THE LEPIDOPTERISTS' NEWS Volume 6 1952 Numbers 1-3 THE STRUCTURE OF THE LARVAL PROLEGS OF T HE LEPIDOPTERA AND THEIR VALUE IN THE CLASSIFICATION OF THE MAJOR GROUPS by H. E. HINTON In 1946 I proposed a new subordinal classification of the Lepidoptera. This classification differed from that of Borner (1939) in two .lmportant particulars: ( 1) the Micropterygidae were placed in a separate order, the Zeugloptera, as first suggested by Chapman (1917), and (2) the suborder Dacnonypha was erected to contain the Eriocraniidae and related families with a decticous pupa (Hinton, 1946a). Each year since then a number of classifications of the Lepidoptera have appeared. Most of these, it must be admitted, are new arrange­ ments produced by reshuffling already known facts. The classification of Kiriakoff (1948), however, deserves especial attention, as he has discovered a number of new facts about the structures of the tympanum. In the not too distant future I hope to reply in detail to the various critics of my classification, particu­ larly as regards the position of the Micropterygidae. In the space now at my disposal I can do no more than reply rather briefly to those who believe that I paid insufficient attention to the structure of the larval prolegs. For instance, Kiriakoff (1948, p. 133) says of the structure of the larval prolegs, "Borner 1939 has rejected it as of no phylogenetic value; nor is it mentioned in Hinton's provisional scheme." The great majority of the Lepidoptera are placed by Kiriakoff in two groups, the Stemmatoncopoda and the Harmoncopoda. To the first group belong all these species that have a complete or nearly complete circle of crochets on the ventral prolegs and to the second those that have a single longitudinal row of crochets. Thus Kiriakoff's division of the majority of the Lepidoptera does not differ from the older classifications of Karsch, Heymons, and others. ~:~ :\; It is interesting to note that in the recent classification by Bourgogne .'~J ' .. '" \.; .,,('1951) no importance is attached to the structure of the prolegs, although Bourgogne still accepts Tilly~rd's two suborders, the Homoneura and the Heteroneura, a division of the Lepidoptera considered to be unnatural by both Borner (1939) and myself (1946). If it be admitted that the primary aim of classification be to show, so far as is ,possible, the natural or genetic relations of the groups within the order, it fonows as a necessary corollary that each group shall be monophyletic in terms of any other. If, therefore, it can be shown that the prolegs of the harmon­ ocopodous type have been independently evolved by two or more families, the basis for the retention of the Stemmatocopoda and Harmonocopoda will have been destroyed, since it will be then apparent that the Harmonocopoda merely include those families in which the structure of the larval prolegs happens to . I Plate I HINTON: LARVAL PROLEGS Lep. News, Vol. 6 3 4 Figs. 1-2 Adela sp. (1) Right side of fourth abdominal segment of full grown larva. (21 Posterior view of left proleg of sixth segment. (mv) Level of mid-ventral line. (s) Level of spiracle. The slender diagonal muscle shown is the occlusor of the spiracle. Figs. 3-4 Polia nebulosa Hufn. (3) Cross section through the fifth abdominal segment to show proleg, which is considerably retracted. ( 4) Inner side of left proleg of fifth abdominal segment as seen when procracted. 1952 The Lepidopterists' N ews 3 converge, and, moreover, will contain families more related to some of those included in the StemmatOcopoda than to others included in the Harmonocopoda. For instance, in Kiriakoff's system the Hesperiidae are placed in the Stemmato· copoda but the remaining families of the Papilionoidea are included 111 the Harmonocopoda. The morphology of the larval prolegs bas received very little attention from a comparative point of view, although the external form of a considerable range of species is described by Goossens (1887). Schultze (1920) has described in some detail the variations in the number and form of the crochets of several species. The many other studies that have been made are restricted ro single genera or families; and apart from isolated accounts of single species and a few very general studies, such as that of Snodgras:; (1935), nothing is known of their internal structure. The ventral prolegs (3-6) are hollow cylindrical outgrowths of the body wall. In most Microlepidoptera (Monotrysia and Ditrysia), and in the early instars of all and the final instar of some Papilionoidea, they have a complete or nearly complete apical circle of strongly sclerotised curved hooks known as crochets. The apical area bounded by and bearing the crochets is membranous and· much less rigid than the sides of the pro legs. It is known as tbe planta. The retracror muscles are inserted in the planta, usually in its centre. They consist of a variable number of separate groups of fibres. In most of the families that were examined, they consist of two distinct sets, one arising well below and the other above the spiracle, as in the Cossidae. In the HepiaJidae (H epialu.r) and Yponomeutidae (Y ponomeuta) , however, all of the planta retractOrs arise above the level of the spiracle. Contraction of the retractOr muscles pulls the centre of the planta inwards, and the whole of the planta may be more or less completely invaginated within the proleg. The crochets on the periphery of the planta have their apices direered outwards and upwards. They are attached to the cuticle of the planta for about one-half to three-fourths of their lengths in such a way that when the planta is pulled inwards against turgor pressure they are so tilted that their apices, instead of projecting upwards, now project ventrally and are more or less parallel to the long axis of the proleg. Thus by retraction of the planta the crochets are disengaged. They are engaged again when the retractors are relaxed and turgor pressure evaginates the planta. The caterpillar therefore always pulls the planta inwards before shifting the position of the proleg. Ripley (1923) claims that in the Phalaenidae muscles are inserted in the proximal ends of the crochets, but this is not so either in that or related families. Species belonging to all of the principal superfamilies of the order have been examined, and in no case were muscles inserted in tbe crochets themselves. Movements of the prolegs, other than retraction or evagination, are effected by variations of turgor pressure and by the muscles inserted in the body wall near their bases. The areas of the body wall adjoining those that give rise to the prolegs have numerous transverse, oblique, and longitudinal muscles as shown, for instance, in fig. l. It is probable that the retractors of the planta are, as in the Diptera, merely slightly specialized muscles of the body wall. Among the most simple ventral prolegs are those of the Adelidae (figs. 1 and 2). Whether this simplicity is brought about by reduction from a more complex proleg or whether it is 4 HINTON: Larval Prolegs Vol. 6, 00s.1-3 primitive will be discussed elsewhere. The prolegs of the Adelidae differ little in structure from the transverse creeping welts of the Diptera, and it seems certain that they resemble a stage in the evolution of prolegs from structures like creeping-welts even though they may not actually be one. The ventral pro legs of the Zygaenidae, many Papilionoidea, Geometridae, Phalaenoidea, Bombycoidea, Saturnioidea, and Sphingidae are modified in a very characteristic fashion that enables them to cling to thin twigs with a force not possible to larvae of comparable size that have prolegs of the primitive type. When the larva is at rest, the twig is clasped between the prolegs in such a way that, if narrow enough, the prolegs may completely embrace it. At first sight it appears that this type of proleg is especially adapted for climbing about on plants. This impression receives support from the fact that it is the type found in all of the exclusively arboreal caterpillars that move about freely exposed and do not live in shelters of some kind as do most of the Microlepidoptera. Furthermore, the only terrestrial or semi-terrestrial larvae that have this kind of proleg are species of Phalaenoidea that can be shown in each case to be secondarily terrestrial. Each of these specialised prolegs is somewhat tilted mesally towards the other of the same pair. The crochets, which do not differ in structure from those of the primitive type of proleg, are restricted to a straight or sinuate longitudinal line on the mesal side of the planta only. This arrangement of crochets is known as a mesoseries, and each crochet has its apex curved mesally and upwards. The planta is retracted and everted as in the Microlepidoptera. The retractor muscles often also consist of two sets, one arising on the body wall directly above the proleg dorsal to the spiracle and the other ventral to the spiracle (figs. 3-4), as in the Zygaenidae, Lycaenidae, Phalaenidae, Arcti­ idae, and Sphingidae. The centre of the planta in which the retractor muscles are inserted is, of course, always laterad from the crochets. As in the primitive proleg, the crochets of the specialised type are dis­ engaged when the planta is pulled inwards, but the action of the proleg as a whole is much more complex. A variable number of muscles are inserted on the base of the proleg itself (figs. 3-4), and in some groups, e. R. the Geomet­ ridae, muscles that arise near the spiracle are inserted on its middle outer face.