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

3 E xternal S tructur e 1 . Intr oduc t ion T he extreme variet y of external form seen in the Insecta is the most obvious manifestation of this g roup’s adaptabilit y . To the taxonomist who thrives on morpholo g ical differences , t his variet y is manna from Heaven; to the morpholo g ist who likes to refer ever y thin g back to a b as i c type or groun d p l an, i t can b ean i g h tmare! Para ll e li ng t hi svar i ety i s, un f ortunate l y , a mass i ve term i no l ogy,event h e b as i cs o f w hi c h an e l ementary stu d ent may fi n d diffi cu l t t oa b sor b . Some conso l at i on ma yb e d er i ve df rom t h e f act t h at “ f orm re fl ects f unct i on.” I n other words, seemin g l y minor differences in structure ma y reflect important difference s in functional ca p abilities. It is im p ossible to deal in a text of this kind with all of th e var i at i on i n f orm t h at ex i sts, an d on l yt h e b as i c structure o f an i nsect an di ts most i mportant mo difi cat i ons will b e d escr ib e d . 2. G eneral Bod y Pla n Like other arthropods insects are se g mented animals whose bodies are covered wit h cuticle. Over most regions of the body the outer layer of the cuticle becomes hardened (tanne d )an df orms t h e exocut i c l e (see C h apter 11, Sect i on 3.3). T h ese reg i ons are sepa- rate db y areas ( j o i nts) i nw hi c h t h e exocut i cu l ar l ayer i sm i ss i ng, an d t h e cut i c l et h ere f ore remains membranous, flexible, and often folded. The presence of these cuticular membranes facilitates movement between ad j acent hard parts ( s clerites). The de g ree of movement at a joint depends on the extent of the cuticular membrane. In the case of intersegmenta l mem b ranes t h ere i s comp l ete separat i on o f a dj acent sc l er i tes, an d t h ere f ore movement i s u nrestr i cte d . Usua ll y, h owever, espec i a ll y at appen d age j o i nts, movement i s restr i cte dby t he development of one or two conti g uous points between ad j acent sclerites; that is, specific articulations are p roduced. A m onocondyli c j oint has onl y one articulator y surface, and at t his j oint movement ma y be partiall y rotar y (e. g ., the articulation of the antennae with th e h ea d ). In d icon dyl ic j o i nts (e.g., most l eg j o i nts) t h ere are two art i cu l at i ons, an d t h e j o i nt operates lik ea hi nge. T h e art i cu l at i ons may b ee i t h e r i ntrin s ic , w h ere t h e cont i guous po i nts li ew i t hi nt h e mem b rane (F ig ure 3.1A), or e xtrin s ic ,i nw hi c h case t h e art i cu l at i n g sur f aces lie outside the skeletal parts (Fi g ure 3.lB) . 5 7 58 CHAPTER 3 F I GU RE 3.1. Articulations. (A) Intrinsic (le gj oint); and (B) extrinsic (articulation of mandible with cranium). [From R. E. Snodgrass , P rinciples o f Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with per- mi ss i on o f McGraw-H ill Boo k Compan y .] In man y larval insects (as in annelids) the entire cuticle is thin and flexible, an d segments are separate db y i nvag i nat i ons o f t h e i ntegument ( intersegmenta lf o lds ) t o whi c h l ong i tu di na l musc l es are attac h e d (F i gure 3.2A). An i ma l s possess i ng t hi s arrangement (k nown a s p rimar y se g mentation )h ave al most un li m i te df ree d om o fb o d y movement. In the ma j orit y of insects, however, there is heav y sclerotization of the cuticle to form a series o f dorsal and ventral p lates, the t erga a n d st ern a , respectivel y . As shown in Fi g ure 3.2B , F IGURE 3.2. Types of body segmentation. (A) Primary; and (B) secondary. [From R. E. Snodgrass , P rinciples o f I nsect Morp h o l ogy . Cop y ri g ht 193 5 b y McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Compan y . ] 59 E XTER NAL STRUCTURE t hese re g ions of sclerotization do not correspond precisel y with the primar y se g mental pattern. The ter g al and sternal plates do not cover entirel y the posterior part of the primar y se g ment, y et the y extend anteriorl y sli g htl y be y ond the ori g inal interse g mental g roove . Th us, t h e b o d y i s diff erent i ate di nto a ser i es o f secon d ary segments ( s c l eromat a ) se p arate d b y mem b ranous areas ( c on j unctiva e ) t h at a ll ow t h e b o d y to rema i n fl ex ibl e. T hi s i s terme d secon d ar y se g mentation . Eac h secon d ar y se g ment conta i ns f our exos k e l eta l components, a ter g um and a sternum separated b y lateral, primaril y membranous , p leural areas . Each of t he primar y components ma y differentiate into several sclerites to which the g eneral term s ter gi tes a n d s tern i te s are app li e d ; sma ll sc l er i tes, genera ll y terme d pl eurites, may a l so occu r i nt h ep l eura l areas. T h epr i m i t i ve i ntersegmenta lf o ld b ecomes an i nterna l r id ge o f cut i c l e, th e anteco s ta, seen externa ll yasagroove,t he a nteco s ta ls u l cu s .T h e narrow str i po f cut i c le anterior to the sulcus is th e a crotergit e ( when dorsal ) or a crosternit e ( when ventraJ ) .Th e posterior part of both the ter g um and sternum is primitivel y a simple cuticular plate, but t his undergoes considerable modification in the thoracic region of the body. The pleurites a re usua ll y secon d ary sc l erot i zat i ons b ut i n f act may represent t h e b asa l segment o f t he a ppen d ages. T h ep l eur i tes may b ecome great l yen l arge d an df use d w i t h t h e tergum an d sternum in the thoracic se g ments. In the abdomen the pleurites ma y fuse with the sternal p lates . The basic segmental structure is frequently obscured as a result of tagmosis. In insect s th ree tagmata are f oun d :t h e h ea d ,t h et h orax, an d t h ea bd omen. In t h e h ea d a l most a ll s i gns o f t h eor i g i na lb oun d ar i es o f t h e segments h ave di sappeare d ,t h oug h , f or most segments , t he appenda g es remain. In the thorax the three se g ments can g enerall y be distin g uished, a lthou g h the y under g o profound modification associated with locomotion. The anterior a bdominal se g ments are usuall y little different from the t y pical secondar y se g ment described ab ove.Att h e poster i or en d o f t h ea bd omen a var i e d num b er o f segments may b emo difi e d , re d uce d ,or l ost, assoc i ate d w i t h t h e d eve l opment o f t h e externa l gen i ta li a. Exam i nat i on o f t h e exos k e l eton revea l st h e presence o f a num b er o fli nes or g rooves w hose ori g in is varied. If the line marks the union of two ori g inall y separate sclerites, it is kno w nas a su t ure .I fi t i ndicates an inva g ination of the exoskeleton to form an internal rid ge o f cut i c l e ( apo d em e ) ,t h e li ne i s proper l y terme da s u l cu s (Snodgrass, 1960). Pits may als o b e seen on t h e exos k e l eton. T h ese p i ts mar k t h es i tes o fi nterna l ,tu b ercu l ar i nvag i nat i ons o f t h e i ntegument ( apop hy se s ) . Secon d ary di scont i nuat i ons o f t h e exocut i cu l ar component of the cuticle ma y occur, for example, the ecd y sial line alon g which the old cuticle splits d urin g moltin g , and these are g enerall y known as sutures. Primitively each segment bore a pair of appendages. Traces of these can still be seen on a l most a ll segments f oras h ort t i me d ur i ng em b ryon i c d eve l opment, b ut on many seg- ments t h ey soon di sappear, an d typ i ca li nsects l ac k a bd om i na l appen d ages on a ll except t he posterior se g ments. Accordin g to Kukalov´a-Peck (1987), the g round plan of the insec t se g mental appenda g e included 11 p odite s , some of which carried inner or outer branches ( endite s a n d e x i te s , respectively) (Figure 3.21A). All of these podites can be identified in some f oss il i nsects (Ku k a l ov´a-Pec k , 1992), b ut i nt h e great ma j or i ty o f extant f orms on l y fi ve or s i xpo di tes at most are o b v i ous, nota bl y i nt h e l egs [ cox a , troc h ante r , f emu r , ti b ia , t ar s u s ( an d p retarsus,w hi c h some aut h ors d o not cons id er to b eapo di te)]. T h e appen d a g es of the head and abdomen have become so hi g hl y modified that homolo g izin g their podite s ma y be extremel y difficult. Traces of exites can be seen as g ills in some aquatic j uvenile i nsects, an d t h een di tes rema i nast h e exsert il eves i c l es o f some apterygotes, b ut i nt h e ma j or i ty o fi nsects t h ese b ranc h es h ave comp l ete l y di sappeare d . 60 CHAPTER 3 3 .Th e H ead Th e h ea d , b e i n g t h e anter i or ta g ma, b ears t h ema j or sense or g ans an d t h e mout h parts . Considerable controvers y still surrounds the problem of se g mentation of the insect head, e speciall y concernin g the number and nature of se g ments anterior to the mouth. * At v ariou s times it has been argued that there are from three to seven segments in the insect head , t h oug hi t i snoww id e l y agree d t h at t h ere are s i x. It i s not f eas ibl eto di scuss h ere t h e man y t h eor i es concern i n g t h ese g menta l compos i t i on o f t h e i nsect h ea d , b ut t h ema i npo i nts o f c ontention should be noted. These are: (1) whether arthropods possess an acron ( which is non-se g mental and homolo g ous with the annelid prostomium); (2) whether a preantenna l s egment occurs between the acron and the antennal segment and what appendages are a ssoc i ate d w i t h suc h a segment; an d (3) w h et h er t he a ntennae a re segmenta l appen d ages o r merely outgrowths of the acron [see Rempel (197 5 ), Bitsch (1994), Kukalov´a-Peck (1992) , a nd Scholtz (1998) for reviews of the sub j ect]. The embr y olo g ical studies of Rempel and C hurch (1971) have demonstrated convincin g l y that an acron is present. However, it is never s een in fossil insects or other arthropods (Kukalov´a-Peck, 1992) because it moved dorsall y to merge i mpercept ibl y i nto t h ereg i on b etween t h e compoun d eyes (Ku k a l ov´a-Pec k , 1998). Bot h em b ryo l ogy an d pa l eonto l ogy h ave con fi rme d t h at t h ere are t h ree preora l an d t h ree p ostoral se g ments The first preoral se g ment is preantennal; it is called the p rotocerebral or clypeolabral s e g ment. The se g ment itself has disappeared but its appenda g es remain as the clypeolabrum . The second p reoral ( a ntennal / deutocerebra l ) s e gment bears th e a n te nn ae , w hi c h are t h ere f ore true segmenta l appen d ages. T h et hi r d preora l (interca l ar y /tritocere b ra l ) s egment appears b r i e fl y d ur i ng em b ryogenes i s, t h en i s l ost. Its appen d ages, h owever, rema i n a s part of the h ypopharynx (Kukalov´a-Peck, 1992). Head se g ments 4– 6 are postoral and n a m ed t h e mandibula r , m axillary , an d labial , respectivel y . Their appenda g es form the mouth p arts from which their names are derived. In addition, the sternum of the mandibular s egment b ecomes part o f t h e h ypop h arynx . 3.1. G eneral S tructure P rimitively the head is oriented so that the mouthparts lie ventrally (the hypognathou s con di t i on) (F i gure 3.3B). In some i nsects, espec i a ll yt h ose t h at pursue t h e i r prey or use t h e i r mout h parts i n b urrow i ng, t h e h ea di s p ro g nat h ou s i nw hi c h t h e mout h parts are di recte d a nteriorl y (Fi g ure 3.4A). In man y Hemiptera the suctorial mouthparts are posteroventral i n p osition (Fi g ure 3.4B), a condition described as opisthognathou s ( o pisthorhynchous ). The head takes the form of a heavily sclerotized capsule, and only the presence of th e a ntennae an d mout h parts prov id es any externa li n di cat i on o fi ts segmenta l construct i on. In most a d u l t i nsects an dj uven il e exopterygotes a pa i ro f compoun d e y e s i ss i tuate dd orso l ater- a ll y on the cranium, with three ocelli b etween them on the anterior face (Fi g ure 3.3A). Th e two p osterior ocelli are somewhat lateral in p osition; the third ocellus is anterior and median. Th e a n te nn ae v ary in location from a point close to the mandibles to a more median positio n b etween t h e compoun d eyes. On t h e poster i or sur f ace o f t h e h ea d capsu l e i s an aperture, t h e occipita l forame n ,w hi c hl ea d s i nto t h e nec k .O f t h e mout h parts, t h e l a b ru m h angs d own f rom the ventral ed g eofth e c lypeus , th e labium l ies below the occipital foramen, and th e * P erhaps the most interesting conclusion was drawn by Snodgrass (1960, p. 51) who stated “it would be too ba d if t h e quest i on o fh ea d segmentat i on ever s h ou ld b e fi na ll y sett l e d ; i t h as b een f or so l ong suc hf ert il e groun d f or t h eor i z i n g t h at art h ropo di sts wou ld m i ss i tasa fi e ld f or menta l exerc i se”! 6 1 E XTER NAL S TRUCTURE F IGURE 3.3. Structure of the typical pterygotan head. (A) Anterior; (B) lateral; (C) posterior; and (D) ventral ( appen d a g es remove d ). [From R. E. Sno dg rass . P rincip l es of Insect Morp h o l ogy . C op y ri g ht 193 5 b y McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Compan y .] p aired mandible s a n d m axillae occup y ventrolateral positions (Fi g ure 3.3B). The mouth is situated behind the base of the labrum. The true ventral surface of the head ca p sule is the h ypop h arynx (F i gure 3.3D), a mem b ranous l o b et h at li es i nt h e preora l cav i ty f orme dby th e ventra ll y pro j ect i ng mout h parts . There are several g rooves and pits on the head (Fi g ure 3.3A–C), some of which, b y virtue of their constanc y of position within a particular insect g roup, constitute importan t t axonomic features. The g rooves are almost all sulci. Th e p ostoccipital sulcus s e p arates the m ax ill ary an dl a bi a l segments an di nterna ll y f orms a strong r id ge to w hi c h are attac h e d th e musc l es use di nmov i ng t h e h ea d an df rom w hi c h t h e poster i or arms o f t h e tentorium ar i se (see f o ll ow i n g para g rap h ). T h epo i nts o ff ormat i on o f t h ese arms are seen externa lly as deep pits in the postoccipital g roove, the p osterior tentorial p it s . Th e e p icranial sutur e 62 CHAPTER 3 F IGURE 3.4 . ( A) Prognathous; and (B) opisthognathous types of head structure. [A, from R. E. Snodgrass, Princip l es of Insect Morp h o l ogy . C op y ri g ht 193 5 b y McGraw-Hill, Inc. Used with permission of McGraw-Hil l B ook Compan y . B, after R. F. Chapman, 1971 , T he Insects: S tructure and Function . B y permission of Elsevie r North-Holland, Inc., and the author. ] i s a line of weakness occupying a median dorsal position on the head. It is also known a s the ec dy sia ll in e , f or i t i sa l ong t hi s groove t h at t h e cut i c l esp li ts d ur i ng ec d ys i s. In man y i nsects t h eep i cran i a l suture i s i nt h es h ape o f an i nverte d Yw h ose arms di verge a b ove t h e m edian ocellus and pass ventrall y over the anterior part of the head. Th e o cci p ital sulcus , w hich is commonl y found in orthopteroid insects, runs transversel y across the posterior part of the cranium. Internall y it forms a rid g e that stren g thens this re g ion of the head. Th e s u bg ena l su l cus i sa l atera l groove i nt h e cran i a l wa ll runn i ng s li g h t l ya b ove t h e mout h part art i cu l at i ons. T h at part o f t h esu b gena l su l cus l y i ng di rect l ya b ove t h e man dibl e i s k nown as t h e pl eurostoma l su l cu s ;t h at part lyi n gb e hi n di st h e hy postoma l su l cu s ,w hi c hi s usua lly c ontinuous with the postoccipital suture. In man y insects the pleurostomal sulcus is contin - ued across the front of the cranium (above the labrum), where it is known as th e epistoma l ( f rontoc l ypea l ) s u l cu s .W ithin this sulcus lie the W W a nterior tentoria l pit s , w hi c hi n di cate t he i nterna l or i g i no f t h e a nterior tentoria l arm s .T he antenna l a n d o cu l ar s u l c i i n di cate i nterna l c ut i cu l ar r idg es b rac i n g t h e antennae an d compoun d e y es, respect i ve ly . A s u b ocu l ar s u l cu s runnin g dorsolaterall y beneath the compound e y e is often present. 63 E XTER NAL S TRUCTURE F I GU RE 3.5 . Relationship of the tentorium to g rooves and pits on the head. Most of the head capsule has been cut away. [From R. E. Snodgrass. P rinciples o f Insect Morphology . Copyright 1935 by McGraw-Hill, Inc. Use d w i t h perm i ss i on o f McGraw-H ill Boo k Compan y .] The tentorium (Fi g ure 3.5) is an internal, cranial-supportin g structure whose mor - pholo gy varies considerabl y amon g different insect g roups. Like the f urc a o f t h et h o r ac i c segments (Section 4.2), with which it is homologous, it is produced by invagination of the exos k e l eton. Genera ll y, i t i s compose d o f t h e anter i or an d poster i or tentor i a l arms t h at ma y meet an df use w i t hi nt h e h ea d . Frequent l y, a ddi t i ona l supports i nt h e f orm o fd orsa l arms are found. The latter are secondar y out g rowths of the anterior arms and not apodemes. Th e j unction of the anterior and posterior arms is often enlar g ed and known as th e te n to r ial b ridg e or c orporotentor i u m . In addition to bracing the cranium, the tentorium is also a site f or t h e i nsert i on o f musc l es contro lli ng movement o f t h e man dibl es, max ill ae, l a bi um, an d h ypop h arynx. The g rooves described above delimit particular areas of the cranium that are useful i n d escriptive or taxonomic work. The ma j or areas are as follows. The f rontoclypeal area i st h e facial area of the head, between the antennae and the labrum. When the e p istomal sulcus is p resent, t h e area b ecomes di v id e di nto t h e d orsa l f rons an d t h e ventra l c l ypeus. T h e l atter is o f ten divid e di nto a p ostc ly peus an d an a ntec ly peus . T h e v ertex i st h e d orsa l sur f ace o f t h e h ea d .It i s usua lly d e li m i te d anter i or ly by t h e arms o f t h eep i cran i a l suture an d poster i or ly by the occipital sulcus. The vertex extends laterall y to mer g e with the gena , whose anterior , posterior, and ventral limits are the subocular, occipital, and sub g enal sulci, respectivel y. Th e h orses h oe-s h ape d area l y i ng b etween t h e occ i p i ta l su l cus an d postocc i p i ta l su l cus i s genera ll y di v id e di nto t h e d orsa l o cciput , w hi c h merges l atera ll yw i t h t h e post g ena e .T he p ostocci p ut i st h e narrow poster i or r i mo f t h e cran i um surroun di n g t h e occ i p i ta lf oramen. I t bears a pair o f o ccipital condyles t o which th e a nterior cervical sclerite s are articulated . B elow the g ena is a narrow area, th e s ubgen a ,on w hich the mandible and maxilla are art i cu l ate d .T h e l a bi um i s usua ll y art i cu l ate ddi rect l yw i t h t h e nec k mem b rane (F i gure 3.3C), b ut in some i nsects a sc l erot i ze d reg i on separates t h etwo.T hi ssc l erot i ze d area d eve l ops i n one o f t h ree ways: as extens i ons o f t h esu b genae w hi c hf use i nt h em idli ne to f orm a subgenal bridge ,ase xtensions of the h y postomal areas to form a hypostomal bridge ,or 64 CHAPTER 3 ( in most pro g nathous heads) throu g h the extension ventrall y and anteriorl y of a ventra l c ervical sclerite to form th e gul a . At the same time the basal se g ment of the labium ma y also become elon g ated (Fi g ure 3.4A) . 3 .2. Head A ppendage s 3 .2.1. A nt e nn ae Apa i ro f antennae are f oun d on t h e h ea d o f t h e pter yg ote i nsects an d t h e apter y - g ote g roups with the exception of the Protura. However, in the larvae of man y hi g he r Hy menoptera and Diptera the y are reduced to a sli g ht swellin g or disc. In a typical antenna (Figure 3.6) there are three principal components: the basal s cap e b yw hi c h t h e antenna i s attac h e d to t h e h ea d ,t h e pe d ice l conta i n i ng Jo h nston’s organ ( C h apter 12, Sect i on 3.1), an d t h e fla g e ll um,w hi c hi s usua lly l on g an d annu l ate d . Accor d - i n g to Kukalov´a-Peck (1992), the scape, pedicel, and fla g ellum are homolo g ous with th e s ubcoxa, coxa, and remainin g se g ments, respectivel y , of the ancestral le g (Fi g ure 3.21A). The annuli on the flagellum do not correspond with the ancestral leg joints; that is, the annuli are constr i ct i ons, not sutures. T h e scape i s set i n a mem b ranous soc k et an d surroun d e dby t h e antenna l sc l er i te on w hi c h as i ng l e art i cu l at i on may occur. In t h ema j or i ty o fi nsects m ovement of the whole antenna is effected b y muscles inserted on the scape and attached to the cranium or tentorium. However, in Collembola there is no Johnston’s or g an and each antennal segment is moved by a muscle inserted in the previous segment. A l t h oug h reta i n i ng t h e b as i c structure out li ne d a b ove, t h e antennae ta k eonaw id e v ar i ety o ff orms (F i gure 3.7) re l ate d to t h e i rvar i e df unct i ons. Genera ll y, i t i st h e fl age ll u m t h at i smo difi e d . For examp l e, i n some ma l e mot h san db eet l es t h e fl a g e ll um i sp l umose an d flabellate, respectivel y , providin g a lar g e surface area for the numerous chemosensilla that g ive these insects their remarkable sense of smell (see Chapter 12, Section 4). B y contrast, t h ep l umose nature o f t h e antennae o f ma l e mosqu i toes ma k es t h em hi g hl y sens i t i ve to t h e s oun d s generate db yt h e b eat i ng o f t h e f ema l e’s w i ngs (C h apter 12, Sect i on 3.1). Ot h e r f unct i ons o f antennae i nc l u d e touc hi n g , temperature an dh um idi t y percept i on, g rasp i n g pre y , and holdin g on to the female durin g matin g (Schneider, 1964; Zacharuk, 1985). Fo r taxonomists, this variet y of form ma y be an important dia g nostic feature. 3 . 2 . 2 . Mouthparts T he mouthparts consist of the labrum, a pair of mandibles, a pair of maxillae, the l abium, and the h y pophar y nx. In Collembola, Protura, and Diplura the mouthparts ar e FI G URE 3.6 . Structure o f an antenna. [From R. E. S nod g rass , Principles o f Insect Morphology . Cop y ri g h t 1 935 by McGraw-Hill, Inc. Used with permission o f McGraw-H ill Boo k Compan y .] 65 E XTER NAL S TRUCTURE F I GU RE 3.7. T y pes of antennae. [After A. D. Imms, 1957 , A General Textbook o f Entomolog y , 9th ed. ( revised b y O. W. Richards and R. G. Davies), Methuen and Co.] enclosed within a cavit y formed b y the ventrolateral extension of the g enae, which fuse in t he midline (the entognathou s condition). In Microcor y phia, Z yg entoma, and Pter yg ota th e mout h parts pro j ect f ree l y f rom t h e h ea d capsu l e, a con di t i on d escr ib e d a s ecto g nat h ou s . Th e f orm o f t h e mout h parts i s extreme l yvar i e d (see b e l ow), an di t i s appropr i ate to d escr ib e fi rst t h e i r structure i nt h e more pr i m i t i ve c h ew i n g con di t i on . Typical Chewing Mouthparts . Inat y pical chewin g insect the labrum (Fi g ure 3.3A ) is a broadly flattened plate hinged to the clypeus. Its ventral (inner) surface is usually mem b ranous an df orms t h e l o b e- lik eep i p h arynx, w hi c hb ears mec h ano- an d c h emosens ill a. T h e man dibl e(F i gure 3.8A) i sa h eav il ysc l erot i ze d , rat h er compact structure h av i ng a l most a l wa y sa di con dyli c art i cu l at i on w i t h t h esu bg ena. Its f unct i ona l area var i es accor d- in g to the diet of the insect. In herbivorous forms there are both cuttin g ed g es and g rindin g surfaces on the mandible. The cuttin g ed g es are t y picall y stren g thened b y the addition o f zi nc, manganese or, rare l y, i ron, i n amounts up to a b out 4% o f t h e d ry we i g h t. In carn i vorou s spec i es t h e man dibl e possesses s h arp l ypo i nte d “teet h ” f or cutt i ng an d tear i ng. In M i croco- r y p hi at h e man dibl e h asas i n gl e art i cu l at i on w i t h t h e cran i um an d , as a resu l t, muc hg reater freedom of movement . Of all of the mouthparts the maxilla (Fi g ure 3.8B) retains most closel y the structur e o f t h epr i m i t i ve i nsectan li m b .T h e b asa l segment i s di v id e db y a transverse li ne o ffl exure i nto two su b segments, a prox i ma l c ar do a n d a di sta l s tipe s . T h e car d o carr i es t h es i ng le con d y l ew i t h w hi c h t h e max ill a art i cu l ates w i t h t h e h ea d . Bot h t h e car d oan d st i pes are , h owever, attached on their entire inner surface to the membranous head pleuron. The stipes 66 CHAPTER 3 F IGURE 3.8 . S tructure o f (A) man dibl e, (B) max ill a, an d (C) l a bi um o f a typ i ca l c h ew i ng i nsect. [Fro m R .E.Sno dg rass , Princip l es of Insect Morp h o l ogy . Cop y ri g ht 193 5 b y McGraw-Hill, Inc. Used with permissio n o f McGraw-Hill Book Company. ] bea r sa n inn er laci n ia a n d oute r gale a , and a m axillary pal p . This basic structure is found i n b ot h apterygotes an d t h ema j or i ty o f c h ew i ng pterygotes, a l t h oug hi n some f orms re d uct i o n or loss of the lacinia, galea, or palp occurs. In Kukalov´ a-Peck’s (1991) view the cardo ´ an d st i pes correspon d to t h esu b coxa an d coxa + troc h anter, respect i ve ly ,o f t h e ancestra l appenda g e; the lacinia and the g alea to the coxal and trochanteral endites, respectivel y ;an d the palp to the remainin g se g ments. The laciniae assist in holdin g and masticatin g the food, whil et h ega l eae an d pa l ps are equ i ppe d w i t h avar i ety o f mec h ano- an d c h emosens ill a . Th e l a bi um (F i gure 3.8C) i s f orme db yt h eme di a lf us i on o f t h epr i m i t i ve appen d age s of t h e postmax ill ar y se g ment, to g et h er w i t h , i n i ts b asa l re gi on, a sma ll part o f t h e sternum o f that se g ment. The labium is divided into two primar y re g ions, a proximal p os t men t u m c orrespondin g to the maxillar y cardines plus the sternal component, and a distal p remen- tum h omo l ogous w i t h t h e max ill ary st i p i tes. T h e postmentum i s usua ll ysu bdi v id e di nto s u b mentum a n d mentum reg i ons. T h e prementum b ears a pa i ro fi nner gl ossae an d apa i ro f o u t e r para gl ossa e , h omo l o g ous w i t h t h e max ill ar yl ac i n i ae an dg a l eae, respect i ve ly ,an da p air of labial p al p s. When the g lossae and para g lossae are fused the y form a sin g le structur e te rm ed t h e ligul a . Ar i s i ng as a me di an, ma i n l y mem b ranous, l o b e f rom t h e fl oor o f t h e h ea d capsu l ean d pro j ect i ng ventra ll y i nto t h e preora l cav i ty i st h e h ypop h arynx (F i gures 3.3D an d 3.9). I t i s f requent l y f use d to t h e l a bi um. In a f ew i nsects ( b r i st l eta il san d may fl y l arvae) a pa i r o f lobes , the superlinguae , which arise embr y onicall y in the mandibular se g ment, becom e associated with the h y pophar y nx. The h y pophar y nx divides the preoral cavit y into anterio r and p osterior s p aces, the u pp er p arts of which are the ciba r ium ( leading to the mouth) and s a l i v arium ( i nto w hi c h t h esa li vary d uct opens), respect i ve l y . Mouth p art Modifications. Th et y p i ca l c h ew i n g mout h parts d escr ib e d a b ove can b e f ound with minor modifications in Odonata, Pleco p tera, the ortho p teroids and blattoids , [...]... (Figure 3. 23) 77 EXTERNAL STRUCTURE FIGURE 3. 22 Structure of the coxa (A) Lateral view; and (B) coxa with a well-developed meron [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] FIGURE 3. 23 Distal part of a leg showing the arolium and claws [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by... (Figure 3. 14) The proboscis 70 CHAPTER 3 FIGURE 3. 13 Head and mouthparts of larval Diptera (A) Diagrammatic section through the retracted head of Tipula; (B) right mandible of Tipula; (C) left maxilla of Tipula; and (D) diagrammatic section through the anterior end of a maggot [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book... CHAPTER 3 FIGURE 3. 31 Structure of the ovipositor (A) Lepisma (Zygentoma); and (B) a typical pterygote insect [A, r after G G E Scudder, 1961, The comparative morphology of the insect ovipositor, Trans R Entomol Soc Lond 1 13: 25–40 By permission of the Royal Entomological Society B, from R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill... FIGURE 3. 19 Diagrammatic cross-sections of the thorax to show the endoskeleton (A) Normal condition; and (B) condition when furca present [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] FIGURE 3. 20 Ventral view of a generalized thoracic sternum [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by... the ability to take up water from the environment FIGURE 3. 34 Stylus and eversible vesicle of a thysanuran Part of the wall of the plate has been removed to show the musculature [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] 89 EXTERNAL STRUCTURE FIGURE 3. 35 Secondary segmental appendages (A) Proleg of a caterpillar;... and the author B, from R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] (see Chapter 12, Section 4.1) As noted earlier, Kukalov´ -Peck (1987) suggested that the a ancestral limb included 11 podites, as well as exites and endites (Figure 3. 21A) Because of fusion of podites with the pleuron or with adjacent podites the... Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] FIGURE 3. 17 Head and mouthparts of Hemiptera (A) Head with the mouthparts separated; and (B) crosssection of the proboscis [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] maxillary bristles are interlocked... may be 71 EXTERNAL STRUCTURE FIGURE 3. 14 Head and mouthparts of the house fly (A) Lateral view of the head with the proboscis extended; and (B) anterodistal view of the proboscis [From R E Snodgrass, Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] FIGURE 3. 15 Mouthparts of the tsetse fly (A) Cross-section; and (B) lateral view of the... Principles of Insect Morphology Copyright 1 935 by McGraw-Hill, Inc Used with permission of McGraw-Hill Book Company.] FIGURE 3. 32 Sting of the honey bee [After R E Snodgrass, 1925 Anatomy and Physiology of the Honey bee McGraw-Hill Book Company.] 87 EXTERNAL STRUCTURE FIGURE 3. 33 Origin and development of the phallic organs (A) Primary phallic lobes; (B) paired penes of Ephemeroptera; and (C) formation... the basic genitalia are derived from a pair of primary phallic lobes, ectodermal outgrowths belonging to the tenth segment (Figure 3. 33A) However, only in Ephemeroptera do they remain as separate lobes, through the posterior tips of which open the gonopores (Figure 3. 33B) In Zygentoma the lobes meet in the midline to form a short, tubular structure, the “ “penis.” The latter is, in fact, a misnomer, . Snodgrass , P rinciples o f Insect Morphology. Copyright 1 935 by McGraw-Hill, Inc. Used with per- mi ss i on o f McGraw-H ill Boo k Compan y .] In man y larval insects (as in annelids) the entire cuticle. o f t h e h ea d an d (B) cross-sect i on o f t h e p roboscis. [From R. E. Snodgrass, Principles o f Insect Morphology. Copyright 1 935 by McGraw-Hill, Inc. Use d w i t h perm i ss i on o f McGraw-H ill Boo k Company.] (thou g h. (B) cross - section of the proboscis. [From R. E. Snodgrass, P rinciples o f Insect Morphology . Copyright 1 935 by McGraw-Hill, I nc. Use d w i t h perm i ss i on o f McGraw-H ill Boo k Compan y . ] m axillary