Entomology 3rd edition - C.Gillott - Chapter 13 pps

13 N ervous an d C h emica l Integration 1 . Introductio n Animals constantly monitor both their internal and their external environment and make th e n ecessar y ad j ustments in order to maintain themselves o p timall y and thus to develo p and r e p ro d uce at t h e max i mum rate. T h ea dj ustments t h e y ma k ema yb e i mme di ate an d o b v i ous, f or exam pl e, fligh t f rom p re d ators, or l on g er-term, f or exam pl e, entr yi nto di a p ause to avo id i mpending adverse conditions. The nature of the response depends, obviously, on the nature of the stimulus. Only very rarely does a stimulus act directly on the effector system; almost alwa y s a stimulus is received b y an a pp ro p riate sensor y structure and taken to the cen- t ra l nervous s y stem, w hi c h “ d eterm i nes” an a pp ro p r i ate res p onse un d er t h ec i rcumstances. Wh en a res p onse i s i mme di ate, t h at i s, ac hi eve di n a matter o f secon d sor l ess, i t i st h e ner- vous system t h at trans f ers t h e message to t h ee ff ector system. Suc h responses are usua ll y t emporary in nature. Delayed responses are achieved through the use of chemical message s ( viz., hormones) and are g enerall y lon g er-lastin g . The nervous and endocrine s y stems of an i n di v id ua l are, t h en, t h es y stems t h at coor di nate t h e res p onse w i t h t h est i mu l us. Sem i o- c h em i ca l s, w hi c h const i tute anot h er c h em i ca l re g u l at i n g s y stem, coor di nate b e h av i or an d d eve l opment among i n di v id ua l s. T h ey compr i se p h eromones ( i ntraspec i fic coor di nators ) and allelochemicals (interspecific coordinators), which include kairomones and allomones. 2 . Nervous S yste m L ik et h at o f ot h er an i ma l s, t h e nervous s y stem o fi nsects cons i sts o f nerve ce ll s (neurons ) an d g li a l ce ll s. Eac h neuron compr i sesace ll b o d y (per ik aryon) w h ere a nuc l eus, many m itochondria, and other organelles are located, and a cytoplasmic extension, the axon , w hich is usually much branched, the branches being known as neurites. Axons may b e lon g , as in sensor y neurons, motor neurons, and p rinci p al interneurons, or ver y short , as i n l oca li nterneurons. O f ten, i nsect neurons are mono p o l ar, l ac ki n g t h e d en d r i t i c tree c h aracter i st i co f verte b rate nerve ce ll s, t h oug hbi po l ar an d mu l t i po l ar neurons d o occur ( Figure 13.1). Motor (efferent) neurons, which carry impulses from the central nervous system, are monopolar, and their perikarya are located within a ganglion. Sensory (afferent) n eurons are usuall y bi p olar but ma y be multi p olar, and their cell bodies are ad j acent t o 4 0 5 4 06 C HAPTER 13 FI G URE 13.1. N eurons f oun di nt h e i n - sect nervous system. Arrows indicate direc- t i on o fi mpu l se con d uct i on. (A) Monopo l ar; ( B) bi p olar; and (C) multi p olar. [After R. F . C hapman, 1971 , The Insects: S tructure and Function. B y perm i ss i on o f E l sev i er / Nort h - Holland, Inc., and the author.] the sense organ. Interneurons (also called internuncial or association neurons) transmi t information from sensory to motor neurons or other interneurons; they may be mono- or bi p olar and their cell bodies occur in a g an g lion. Interneurons ma y be interse g mental and b ranc h e d ,sot h at t h evar i et y o fp at h wa y sa l on g w hi c hi n f ormat i on can trave l an d ,t h ere f ore, t h evar i ety o f responses are i ncrease d. N eurons are not directly connected to eachotherortothe effector organ but are separate d by a minute space, the synapse or neuromuscular junction, respectively. Impulses may be transferred across the s y na p se either electricall y or chemicall y (Section 2.3). The norma l diameter of axons is 5 µ mor µ µ l ess ;h owever , some i nterneurons w i t hi nt h e ventra l nerv e c or d ,t h e so-ca ll e d “ gi ant fi b ers,” h ave di ameters u p to 6 0 µ m. These giant fibers may run µµ t h e l engt h o f t h e nerve cor d w i t h out synaps i ng an d are un b ranc h e d except at t h e i r term i n i . T hey are well suited, therefore, for very rapid transmission of information from sense orga n to effector or g an; that is, the y facilitate a ver y ra p id but stereot yp ed res p onse to a stimulu s an df or some i nsects are i m p ortant i n esca p e react i ons (Ho yl e, 1974; R i tzmann, 1984). N eurons are a gg re g ate di nto nerves an dg an gli a. Nerves i nc l u d eon ly t h e axona l com - p onent o f neurons, w h ereas gang li a i nc l u d e axons, per ik arya, an dd en d r i tes. T h e typ i ca l structures of a ganglion and interganglionic connective are shown in Figure 13.2. In a gan - g lion there is a central neuro p ile that com p rises a mass of efferent, afferent, and associatio n axons. Fre q uent ly v i s ibl ew i t hi nt h e neuro pil e are g rou p so f axons runn i n gp ara ll e l , k nown as fi b er tracts. T h e p er ik ar y ao f motor an d assoc i at i on neurons are norma lly f oun di nc l uster s a dj acent to t h e neurop il e . Surrounding the neurons are glial cells, which are differentiated according to their p osition and function. The peripheral glial (perineural) cells, which form the perineurium , 4 07 NERV O U S AND C HEMI CA L INTE G RATI ON F I G URE 1 3 .2 . C ross-sect i ons t h rou gh (A) a bd om i na lg an gli on an d (B) i nter g an gli on i c connect i ve to s h o w g eneral structure. [A, after K. D. Roeder, 1963 , Nerve Cells and Insect Behaviour .By p ermission of Harvar d U niversity Press. B, after J.E. Treherne and Y. Pichon, 1972, The insect blood-brain barrier , A dv. Insect Physiol. 9 :2 5 7–313. B yp ermission of Academic Press Ltd., London, and the authors.] are ver y c l ose ly assoc i ate dby t igh t j unct i ons, f orm i n g t h e bl oo d - b ra i n b arr i er (Car l son e t al., 2000; Kretzsc h mar an d P fl ug f e ld er, 2002). T hi s b arr i er i scr i t i ca li n i so l at i ng t h e nervous system from the hemolymph whose composition is both highly variable and inappropriate for neuronal function (see Chapter 17, Section 4). However, the barrier itself creates tw o p otential p roblems, namel y , obtainin g ade q uate su pp lies of ox yg en and nutrients for th e n eura l e l ements. T h e f ormer i sso l ve dbyh av i n g trac h eae runn i n gd ee ply i nto t h e g an gli a , th e l atter by t h ea bili t y o f t h e p er i neura l ce ll s to trans f er mater i a l s b etween t h e h emo ly m ph and neurons. In addition, they secrete the neural lamella, a protective sheath that contain s collagen fibrils and mucopolysaccharide. The lamella is freely permeable, enabling the p erineural cells to accumulate nutrients from the hemol y m p h. The inner g lial cells occur amon g t h e p er ik ar y a i nto w hi c h t h e y exten d fin g er lik e extens i ons o f t h e i rc y to pl asm, t h e t ro ph os p on gi um (F ig ure 13.3A). T h e f unct i on o f t h ese ce ll s i s to trans p ort nutr i ents f ro m per i neura l ce ll stot h e per ik arya. Once i nt h e per ik arya, nutr i ents are transporte d to t h e ir site of use by cytoplasmic streaming . W ra pp ed around each axon or g rou p s of smaller axons are other g lial (Schwann) cell s ( F ig ure 13.3B), T h ese ce ll se ff ect i ve ly i so l ate axons f rom t h e h emo ly m ph i nw hi c h t h e y are b at h e d , However, i n contrast to t h es i tuat i on i n verte b rates, t h e gli a l ce ll s are not com p acte d t o f orm a mye li ns h eat hb ut rat h er are l oose l y woun d aroun d t h e axons, Furt h er, i n i nsec t n erves there are no distinct nodes of Ranvier (the regions between adjacent glial cells); h ence, saltator y conduction of im p ulses does not occur (Section 2.3) . 4 0 8 C HAPTER 13 F I G URE 13.3 . ( A) Ce ll b o dy o f motor neuron s h ow i n g tro ph os p on gi um; an d (B) cross-sect i on t h rou gh axons and surrounding Schwann cells. [A, after V. B. Wigglesworth, 1965, The Principles of Insect Physiology, 6th ed., M et h uen an d Co. By perm i ss i on o f t h e aut h or.B,a f ter J. E. Tre h erne, an d Y. P i c h on, 1972, T h e i nsect bl oo d - b ra in b arr i er , Ad v. Insect P hy sio l . 9 :2 5 7–313 . B yp erm i ss i on o f Aca d em i c Press Lt d ., Lon d on, an d t h e aut h ors.] Structura lly ,t h e nervous s y stem ma yb e di v id e di nto (1) t h e centra l nervous s y stem an di ts p er iph era l nerves an d (2) t h ev i scera l nervous s y stem. 2.1. Central Nervous S y stem T he central nervous system arises during embryonic development as an ectoderma l delamination on the ventral side (Chapter 20, Section 7.3). Each embryonic segment in - c ludes initiall y a p air of g an g lia, thou g h these soon fuse. In addition, var y in g de g rees o f 4 0 9 NERV O U S AND C HEMI CA L INTE G RATI ON F IGURE 1 3 .4. (A) Latera l v i ew o f anter i or centra l nervous system, stomatogastr i c nervous system, an d en - d ocr i ne gl an d so f at ypi ca l acr idid ; (B) di a g rammat i c d orsa l v i ew o fb ra i nan d assoc i ate d structures to s h o w p aths of neurosecretor y axons and relationshi p of cor p ora cardiaca and cor p ora allata; (C) dorsal view of cor p ora car di aca to s h ow di st i nct storage an d g l an d u l ar zones; an d (D,E) transverse sect i ons t h roug h corpora car di aca at l eve l sa– a  an db – b  , res p ectivel y . [A, after F. O. Albrecht, 19 5 3, T h e Anatom y o f t h e Migrator y Locust. By p ermission of The Athlone Press. B–E, after K. C. Hi g hnam, and L. Hill, 1977, The Comparative Endocrinolog y o f t h e Invertebrate s ,2n d e d . By perm i ss i on o f E d war d Arno ld Pu bli s h ers Lt d .] anteroposterior fusion occur so that composite ganglia result. Thus, in an adult insect th e central nervous system comprises the brain, subesophageal ganglion, and a varied numbe r of ventral g an g lia. T h e b ra i n(F ig ure 13.4A) i s p ro b a bly d er i ve df rom t h e g an gli ao f t h ree se g ments an d f orms t h ema j or assoc i at i on center o f t h e nervous system. It i nc l u d es t h e protocere b rum, d eutocerebrum, and tritocerebrum. The protocerebrum, the largest and most complex region of the brain, contains both neural and endocrine (neurosecretory) elements. Anteriorly it forms the p roximal p art of the ocellar nerves (the onl y occasion on which the cell bodies o f sensor y neurons are l ocate d ot h er t h an a dj acent to t h e sense or g an), an dl atera lly is f use d w i t h t h eo p t i c l o b es. W i t hi nt h e p rotocere b rum i sa p a i ro f cor p ora p en d uncu l ata, t he mushroom bodies, so-called because of their outline in cross-section. The mushroom b odies are important association centers, receiving sensory inputs, especially olfactory and visual, and rela y in g the information to other p rotocerebral centers (Strausfeld e ta l. , 1998 ; G ronen b er g , 2001). Furt h er, t h e ypl a y a centra l ro l e i n l earn i n g an d memor y (Sect i on 2.4), an d t h e i rs i ze can b e b roa dly corre l ate d w i t h t h e d eve l o p ment o f com pl ex b e h av i or p atterns. T h ey are most hi g hl y d eve l ope di nt h e soc i a l Hymenoptera. In wor k er ants, f or examp l e , t hey make up about one-fifth the volume of the brain. The median central body is an other important association center, one function of which appears to be the coordinatio n 41 0 C HAPTER 13 F IGURE 13.4 . ( Continued ) o f segmental motor activities, for example, respiratory movements, walking, and flight. Recentl y , the central bod y and the closel y associated p rotocerebral brid g e have been shown to p ossess p o l ar i ze d ligh t-sens i t i ve i nterneurons, su gg est i n g aro l e f or t h ese centers i n nav ig at i on (V i tzt h um e t al. , 2002). Eac h o p t i c l o b e conta i ns t h ree neuro pil ar masses i n w hich light stimuli, including those generated by polarized light, are assessed and forwarde d to other brain centers. T he deutocerebrum is lar g el y com p osed of the p aired antennal lobes (Homber g et al . , 1989; Hannson an d Anton, 2000). T h ese two neuro pil es i nc l u d e b ot h sensor y an d motor neurons an d are res p ons ibl e f or i n i t i at i n gb ot h res p onses to antenna l st i mu li ,es p e - ci a ll yo lf actory an d mec h anosensory, an d movements o f t h e antennae. In spec i es w h ere f emales produce sex-pheromones the antennal lobes often show sexual dimorphism, being l ar g er with additional interneurons in males. From the antennal lobes, interneurons conve y 4 1 1 NERV O U S AND C HEMI CA L INTE G RATI ON i nformation to association centers in both the protocerebrum and thoracic ganglia. Togethe r w ith the mushroom bodies, the antennal lobes are essential in learned olfactor y behavior. T h e trans f er o f mec h anosensor yi n p uts to t h e ventra lg an gli a i s lik e ly re l ate d to p erce p t i o n an d avo id ance o f o bj ects encountere dd ur i n g wa lki n g. T h etr i tocere b rum i s a sma ll reg i on o f t h e b ra i n l ocate db eneat h t h e d eutocere b rum an d comprises a pair of neuropiles that contain axons, both sensory and motor, leading to/fro m t he frontal g an g lion and labrum . T h esu b eso ph a g ea lg an gli on i sa l so com p os i te an di nc l u d es t h ee l ements o f t h eem b r y- on i c g an gli ao f t h e man dib u l ar, max ill ar y ,an dl a bi a l se g ments. From t hi s g an gli on, nerve s conta i n i ng b ot h sensory an d motor axons run to t h e mout h parts, sa li vary g l an d s, an d nec k. The ganglion also appears to be the center for maintaining (though not initiating) locomoto r activity . I n most i nsects t h et h ree se g menta l t h orac i c g an gli a rema i nse p arate. T h ou gh d eta ils v ar yf rom s p ec i es to s p ec i es, eac hg an gli on i nnervates t h e l e g an d fligh t musc l es ( di rect an d i n di rect), sp i rac l es, an d sense organs o f t h e segment i nw hi c hi t i s l ocate d. The maximum number of abdominal ganglia is eight, seen in the adult bristletai l M ac h i l is and larvae of many species, though even in these insects the terminal ganglio n i s com p osite, includin g the last four se g mental g an g lia of the embr y onic sta g e. Var y in g d e g rees o ff us i on o f t h ea bd om i na lg an gli a occur i n diff erent or d ers an d somet i mes t h ere is f us i on o f t h e compos i te a bd om i na l gang li on w i t h t h e gang li ao f t h et h orax to f ormas i ng le t horacoabdominal ganglion. (Chapters 5 –10 contain the details for individual orders. ) 2 .2. Visceral Nervous S y stem T h ev i scera l (s y m p at h et i c) nervous s y stem i nc l u d es t h ree p arts: t h e stomato g astr ic system, t h e unpa i re d ventra l nerves, an d t h e cau d a l sympat h et i c system. T h e stomatogastr ic system, shown partially in Figure 13.4, arises during embryogenesis as an invagination of t he dorsal wall of the stomodeum. Generall y , it includes the frontal g an g lion, recurrent nerv e whi c hli es me di o d orsa lly a b ove t h e g ut, hyp ocere b ra lg an gli on, a p a i ro fi nner eso ph a g ea l n erves, a p a i ro f outer eso ph a g ea l ( g astr i c) nerves, eac h o f w hi c h norma lly term i nates in an i ng l uv i a l (ventr i cu l ar) gang li on s i tuate d a l ongs id et h e poster i or f oregut, an d var i ous fin e n erves from these ganglia that innervate the foregut and midgut, and, in some species, th e h eart. A single median ventral nerve arises from each thoracic and abdominal ganglion i n some insects. The nerve branches and innervates the s p iracle on each side. In s p ecie s wh ere t hi s nerve i sa b sent, p a i re dl atera l nerves f rom t h ese g menta lg an gli a i nnervat e th esp i rac l es. T h e cau d a l sympat h et i c system, compr i s i ng nerves ar i s i ng f rom t h e compos i t e t erminal abdominal ganglion, innervates the hindgut and sexual organs. Nerves withi n t he stomatogastric system both collect mechanosensory and chemical information from , and re g ulate the muscular activit y of, the or g ans the y su pp l y . In the frontal g an g lion, a t l east, t h e neuro pil e h as a centra lp attern g enerator (Sect i on 2.3) t h at contro l sr hy t h m ic m otor act i v i t y o f t h e f ore g ut (A y a li e t al. , 2002) . 2 .3. Physiology of Neural Integration As note di nt h e Intro d uct i on to t hi sc h a p ter, an i nsect’s nervous s y stem i s constant ly r ece i v i n g st i mu li o f diff erent ki n d s b ot hf rom t h e externa l env i ronment an df rom w i t hi n i ts own b o d y. T h esu b sequent response o f t h e i nsect d epen d sont h e net assessment o f t hese stimuli within the central nervous system. The processes of receiving, assessing, an d 412 C HAPTER 13 F IGURE 1 3 .5. C ross-sect i on to s h ow ma j or areas o fb ra i n. [A f ter R. F. C h apman, 1971, Th e Insects: S tructure a n d Function . B yp erm i ss i on o f E l sev i er / Nort h -Ho ll an d , Inc., an d t h e aut h or. ] respon di ng to st i mu li co ll ect i ve l y const i tute neura li ntegrat i on. Neura li ntegrat i on i nc l u d es, therefore, the biophysics of impulse transmission along axons and across synapses, the refle x p athways (in insects, intrasegmental) from sense organ to effector organ, and coordinatio n o f these se g mental events within the central nervous s y stem. Im p u l se transm i ss i on a l on g axona l mem b ranes an d across s y na p ses a pp ears to b e es- sent i a ll yt h e same as i not h er an i ma l san d w ill not b e di scusse dh ere i n d eta il . However, t h e absence of a myelin sheath and nodes of Ranvier precludes the phenomenon of saltatory c onduction seen in vertebrates. Following the arrival of a stimulus of sufficient magnitude, an action p otential is g enerated and the im p ulse travels alon g the axon as a wave of de - p o l ar i zat i on. T h es p ee d o fi m p u l se transm i ss i on i sa f unct i on o f axona ldi ameter so t h at i n gi ant axons va l ues o f 3–7 m p er sec h ave b een recor d e d w hil e i n avera g e-s i ze d axon s the speed is 1. 5 –2.3 m per sec. In addition to “spiking” neurons (i.e., those in which a n action potential can be generated), there are in the insect central nervous system intragan - g lionic “non-s p ikin g ” interneurons unable to p roduce action p otentials. Rather, the amount of neurotransm i tter re l ease d at t h e i rs y na p ses (see b e l ow) i s p ro p ort i ona l to t h es i ze o f t h e ir en d o g enous mem b rane p ermea bili t y c h an g es; i not h er wor d s, t h e y re l ease neurotransm i tter (an d a ff ect t h e postsynapt i c neuron) i n a gra d e d manner. T h ese non-sp iki ng i nterneuron s may have wide importance in the initiation of rhythmic behaviors such as walking, swim - min g , and chewin g (see below) . T ransm i ss i on across a s y na p se, d e p en di n g as i t d oes on diff us i on o f mo l ecu l es t h rou gh fl uid, is relativel y slow and ma y take u p about 25% of the total time for conduction of an i mpu l se t h roug h are fl ex arc. Rare l y, w h en a synapt i c gap i s narrow ( i .e., pre- an d p ostsynaptic membranes are closely apposed), the ionic movements across the presynaptic membrane are sufficient to directly induce depolarization of the postsynaptic membran e 4 1 3 NERV O U S AND C HEMI CA L INTE G RATI ON ( Huber, 1974). Mostly, however, when an impulse reaches a synapse, it causes release of a chemical ( a neurotransmitter ) from membrane-bound vesicles. The chemical diffuses across t h es y na p se an d , i nexc i tator y neurons, b r i n g sa b out d e p o l ar i zat i on o f t h e p osts y - n a p t i c mem b rane. Acet yl c h o li ne i st h e p re d om i nant neurotransm i tter lib erate d at exc i tator y synapes, i nc l u di ng t h ose o fi nterneurons an d a ff erent neurons f rom mec h anosens ill aan d t aste sensilla (Homberg, 1994). 5 -Hydroxytryptamine (serotonin), histamine, octopamine, and do p amine function as central nervous s y stem excitator y neurotransmitters in s p ecific s i tuat i ons on occas i on. T h ese, an d ot h er am i nes, h aveanexc i tator y e ff ect w h en a ppli e din l ow concentrat i ons to t h e h eart, g ut, re p ro d uct i ve tract, etc., an di tma yb et h at t h e y a l so serve as neurotransm i tters i nt h ev i scera l nervous system. S ometimes a single nerve impulse arriving at the presynaptic membrane does not stimulate the release of a sufficient amount of neurotransmitter. Thus, the magnitude o f d e p o l ar i zat i on o f t h e p osts y na p t i c mem b rane i s not l ar g e enou gh to i n i t i ate an i m p u l se in th e p osts y na p t i c axon. I f a ddi t i ona li m p u l ses reac h t h e p res y na p t i c mem b rane b e f ore t he first d epo l ar i zat i on h as d ecaye d ,su f fic i ent a ddi t i ona l neurotransm i tter may b ere l ease d so t hat the minimum level for continued passage of the impulse (the “threshold” level) is ex- ceeded. This additive effect of the presynaptic impulses is known as temporal summation . A second form of summation is s p atial, which occurs at conver g ent s y na p ses. Here, several sensor y axons s y na p se w i t h one i nternunc i a l neuron. A p osts y na p t i c i m p u l se i s i n i t i ate d on l yw h en i mpu l ses f romasu f fic i ent num b er o f sensory axons arr i ve at t h e synapse s i - m ultaneously. Divergent synapses are also found where the presynaptic axon synapses with several postsynaptic neurons. In this arrangement the arrival of a single impulse at a synapse m a y be sufficient to initiate im p ulse transmission in, sa y , one of the p osts y na p tic neurons . T h e arr i va l o f a ddi t i ona li m p u l ses i n q u i c k success i on w ill l ea d to t h e i n i t i at i on o fi m p u l ses i not h er p osts y na p t i c neurons w h ose t h res h o ld l eve l s are high er. T h us, s y na p ses pl a y a n i mportant ro l e i nse l ect i on o f an appropr i ate response f orag i ven st i mu l us . Eventually, an impulse reaches the effector organ, most commonly muscle. Betwee n t he ti p of the motor axon and the muscle cell membrane is a fluid-filled s p ace, com p arable t o a sy na p se, ca ll e d a neuromuscu l ar j unct i on. A g a i n, to ac hi eve d e p o l ar i zat i on o f t h e musc le ce ll mem b rane an d ,u l t i mate ly , musc l e contract i on,ac h em i ca l re l ease df rom t h et ip o f t h e axon diff uses across t h e neuromuscu l ar j unct i on. In i nsect s k e l eta l musc l e, t hi sc h em i ca lis L -glutamate; in visceral muscles, glutamate, serotonin, and the pentapeptide proctolin have all been suggested as candidate neurotransmitters. I n addition to stimulator y (excitator y ) neurons, inhibitor y neurons whose neurotrans - mi tter causes hyp er p o l ar i zat i on o f t h e p osts y na p t i core ff ector ce ll mem b rane are a l so i m p or - t ant i n neura li ntegrat i on. W h en i n hibi t i on occurs at a synapse w i t hi nt h e centra l nervous sys - t em, it is known as central inhibition. Central inhibition is the prevention of the normal stimu- latory output from the central nervous system and may arise spontaneously within the system or result from sensor y in p ut. For exam p le, co p ulator y movements of the abdomen in the mal e m ant i s, w hi c h are re g u l ate dby ase g menta l re fl ex p at h wa yl ocate d w i t hi nt h e term i na l a b - d om i na lg an gli on, are norma lly i n hibi te dby s p ontaneous i m p u l ses ar i s i n g w i t hi nt h e b ra in and passing down the ventral nerve cord . In the fl y P roto ph ormi a t he stimulation of stretch r eceptors during feeding results in decreased sensitivity to taste caused by central inhibitio n of the p ositive stimuli received b y the brain from the tarsal chemorece p tors. When inhibitio n o f an e ff ector or g an occurs i t i s k nown as p er iph era li n hibi t i on. At b ot h s y na p ses an d neuro - m uscu l ar j unct i ons, t h e hyp er p o l ar i z i n g c h em i ca lis γ - am i no b ut y r i cac id (Hom b er g , 1994) . M ent i on must a l so b ema d eo f neuromo d u l ators, a group o f c h em i ca l st h at can m odify the effects of neurotransmitters (Orchard, 1984; Homberg, 1994). Typically, 41 4 C HAPTER 13 neuromodulators are released from the tip of an adjacent neuron (less commonly as a neurohormone released into the hemol y m p h) and act on the p res y na p tic or p osts y na p ti c mem b rane a dj acent to, b ut not w i t hi n, t h es y na p t i c g a p or neuromuscu l ar j unct i on. T h e ir e ff ects i nc l u d ere d uct i on i nt h e amount o f neurotransm i tter re l ease d an di n hibi t i on o f t h eac - t i on o f t h e neurotransm i tter. Am i nes, espec i a ll y octopam i ne, an d some neuropept id es (e.g. , p roctolin) are likely to be important neuromodulators, though in many instances definitive evidence is still lackin g .A p robable neuromodulator of a s p ecial kind ma y be nitric oxide. Thi sver y s h ort- li ve d ,ra pidly diff us i n gg as was di scovere di n nervous t i ssues o fl ocusts , h one yb ees, an d Droso p hil a i nt h e ear ly 1990s. Pro d uct i on o f n i tr i cox id e i ses p ec i a lly r i c h i n i nterneurons i nt h e antenna l an d opt i c l o b es, as we ll as i n antenna l c h emosensory ce ll s o f some species, following appropriate olfactory and visual stimulation, suggesting that this unconventional neuromodulator may have roles in olfactory information processing, olf actor y memor y ,an d v i s i on (M¨u ll er, 1997; B i c k er, 1998). In i nsects re fl ex res p onses are se g menta l ,t h at i s,ast i mu l us rece i ve dby a sense or g a n i n a part i cu l ar segment i n i t i ates a response t h at trave l sv i aan i nterneuron l ocate di nt h at segment’s ganglion to an effector organ in the same segment. This is easily demonstrated b y isolating individual segments. For example, in an isolated thoracic segment preparation of a g rassho pp er, touchin g the tarsus causes the le g to make a ste pp in g movement. Of course , i nan i ntact i nsect suc h ast i mu l us a l so l ea d stocom p ensator y movements o f ot h er l e g sto ma i nta i n b a l ance or to i n i t i ate wa lki ng, act i v i t i es t h at are coor di nate d v i a assoc i at i on centers in the subesophageal ganglion. Touching the tip of the isolated ovipositor i n Bomby x ,fo r e xample, initiates typical egg-laying movements, provided that the terminal ganglion an d its nerves are intact. In other words, each se g mental g an g lion p ossesses a g ood deal of refle x au t onom y . N ervous act i v i t y o f t h et yp e d escr ib e d a b ove, w hi c h occurs on ly a f ter an a pp ro p r i at e st i mu l us i sg i ven, i ssa id to b e exogenous. However, an i mportant component o f nervous activity in insects is endogenous, that is, does not require sensory input but is based o n neurons with intrinsic p acemakers. Such neurons (non-s p ikin g neurons) p ossess s p ecialized mem b rane re gi ons t h at un d er g o p er i o di c, s p ontaneous c h an g es i nexc i ta bili t y ( p ermea bili t y ) an d w h ere i m p u l ses are t h ere by i n i t i ate d .Aw id evar i et y o f motor res p onses are or g an i ze d, i n part, b yen d ogenous act i v i ty. For examp l e, vent il at i on movements o f t h ea bd omen are initiated by endogenous activity in individual ganglia. Even walking and stridulation ar e motor responses under partially endogenous control (Huber, 1974). An obvious question to ask, therefore, is “Wh y don’t insects walk or stridulate continuousl y ?” The answer i s t h at t h ese an d a ll ot h er motor res p onses are “contro ll e d ” by high er centers, s p ec i fica lly t h e b ra i nan d/ or su b esop h agea l gang li on. T h ese assoc i at i on centers assess a ll i n f ormat i o n c oming in via sensory neurons and, on this total assessment, determine the nature of the response. In addition, the centers coordinate and modify identical segmental activities , such as ventilation movements, so that the y o p erate most efficientl y under a g iven set of c on di t i ons . E ar ly ev id ence f or t h ero l eo f t h e b ra i nan d su b eso ph a g ea lg an gli on as coor di nat i n g c enters came from fairly crude experiments in which one or both centers were removed and the resultant behavior of an insect observed. More recent experiments involving localized destruction or stimulation of p arts of these centers have confirmed and added si g nificantl y to t h e g enera lpi cture o b ta i ne dby ear li er aut h ors. To ill ustrate t h e com pl ex i t y o f coor di- nat i on an d contro l o f motor act i v i t y ,wa lki n g w ill b e use d as an exam pl e. T hi sr hy t h m ic stepp i ng movement o f eac hl eg i s contro ll e db y a networ k o f non-sp iki ng neurons (ca ll e d the central pattern generator and located in each half ganglion) whose endogenous activit y [...]... juvenile hormone, metamorphosis-inhibiting hormone, or neotenin, with reference to its function in juvenile insects (Chapter 21, Section 6.1), and gonadotropic hormone to indicate its function in adults (Chapter 19, Sections 3.1.3 and 3.2) Juvenile hormone is a terpenoid compound (Figure 13. 6A) and, to date, six naturally occurring forms (JH-O, JH-I, 4-methyl-JH-I, JH-II, JH-III, and JHB3 ) have been identified... substance, 9-oxo-2-decenoic acid (Figure 13. 7B), which is spread over the body during grooming to be later licked off by attendant workers Mutual feeding among workers results in the dispersal of queen substance through the colony where the pheromone serves to stimulate foraging activity (in older workers) and “household duties” (by young workers) The glands also produce a volatile pheromone, 9-hydroxy-2-decenoic... glands (Chapter 21, Section 6.1); allatotropic and allatostatic hormones, whose primary function is to regulate the activity of the corpora allata (Chapter 19, Section 3.1.3 and Chapter 21, Section 6.1); diuretic hormone, which affects osmoregulation (Chapter 18, Section 5); ovarian ecdysiotropic hormone (OEH) (formerly egg development neurosecretory hormone) (Chapter 19, Section 3.1.3); ovulation- or... for phytophagous insects, for which plant-released chemicals may be a major deterrent against insects attack or a specific cue by which the insect recognizes its host (Chapter 23, Section 2.3.1) In this chapter only pheromones, and kairomones and allomones released by insects will be considered 421 NERVOUS AND CHEMICAL INTEGRATION 422 4.1 Pheromones CHAPTER 13 Pheromones are chemicals messages produced... aliphatic straight-chain hydrocarbons, alcohols, acetates, aldehydes, and ketones containing 10–21 carbon atoms (Figure 13. 7A) Among Lepidoptera, species in the same family or subfamily tend to produce a “key component.” For example, (Z )-1 1-tetradecenol or its derivative is produced by almost all Tortricinae Usually the sex pheromone is a blend of two or more components, occurring in species-specific proportions... This may function, like those of females, as a long-distance “attractant” or may trigger close-in behavior such as short-range attraction, female orientation for mating, adoption of the mating posture, and quiescence For example, the male cockroach, Nauphoeta cinerea, produces seducin, which both attracts and pacifies the unmated female 424 CHAPTER 13 so that a connection can be established The pheromone... Hemiptera, Lepidoptera, and higher Diptera, only JH-III has been obtained Though JH-III is reputedly synthesized in some Hemiptera, Numata et al (1992) report that JH-I is the only form produced in the bean bug, Riptortus clavatus In Lepidoptera the first five forms of JH listed * In many Lepidoptera, including Bombyx, egg development begins in the pupa 420 CHAPTER 13 above occur, with one or two forms predominating... which is 427 NERVOUS AND CHEMICAL INTEGRATION 428 CHAPTER 13 identical to the cuticular hydrocarbons, enables returning foragers to locate the nest under the low light conditions of the cavity (Steinmetz et al., 2002, 2003) Among non-social insects, trail-marking pheromones are well known in cockroaches and gregarious caterpillars For example, the trail-marking pheromone of P americana serves to aggregate... 57:121 131 Lefevere, K S., Lacey, M J., Smith, P H., and Roberts, B., 1993, Identification and quantification of juvenile hormone biosynthesized by larval and adult Australian sheep blow fly Lucilia cuprina (Diptera: Calliphoridae), Insect Biochem Mol Biol 23: 713 720 Lehrer, M., 1991, Bees which turn back and look, Naturwissenschaften 78:274–276 Lehrer, M., and Bianco, G., 2000, The turn-back-and-look... females, the attractants produced by males are usually long-chain alcohols or their aldehydic derivatives Because of their specificity and effectiveness in very low concentrations, the use of sex attractants in pest control has great potential, an aspect that will be more fully discussed in Chapter 24, Section 4.2 Sexual maturation-accelerating or -inhibiting pheromones are produced by many insects that . (EH), i mportant i nec d ys i s (Chapter 21, Section 6.2), is produced by neurosecretory cells in the tritocerebrum. Th e intrinsic cells of the corpora cardiaca produce hyperglycemic and adipokinetic hormone s (AKH). ovu l at i on- or ov ip os i t i on- i n d uc i n g hormone (Chapter 19, Sections 5 and 7.2); and testis ecdysiotropin (TE) (Chapter 19, Sec- tion 3.2). Bursicon, which is important in cuticular tanning. aga i nst i nsects attac k or a spec i c cue b yw hi c h t h e i nsect recogn i ze s i ts host (Chapter 23, Section 2.3.1). In this chapter only pheromones, and kairomones and allomones released b y insects