257 



INSECTA. 



INSECTS. 



258 



Fig. 34. 



Figs. 31, 32, 33, and 34, Eggs of various insects. 



embryonic, so that it is usually not until a series of very considerable 

 changes have taken place in external configuration and internal struc- 

 ture, together constituting what ia known as the Metamorphosis that 

 the complete development of the specific type is attained. The 

 amount of this metamorphosis, and the mode in which it is accom- 

 plished, vary considerably in the different orders of insects ; but 

 these stages are usually marked out more or less distinctly in the life 

 of each individual. The term ' Larva,' in the ordinary language of 

 Entomology, is applied to the insect from the date of its escape from 

 the egg, up to the time when the wings begin to appear ; the term 

 ' Pupa ' is in like manner employed to mark the period during which 

 it is acquiring wings ; and from the time when these and other organs 

 characteristic of its perfect state are complete, it is spoken of as the 

 ' Imago." The grade of development however at which the insect 

 comes forth from the egg is very different in the several orders and 

 families. In all cases the embryonic mass within the egg is first con- 

 verted into a footless worm, resembling the higher Entozoa or the 

 inferior Annelida in its general organisation, but possessing the 

 number of segments, thirteen, which is typical of the class of Insects. 

 Such, in the Diptera and llymenoptera, and in some of the Coleoptera, 

 ia the condition of the larva at the time of its escape from the egg ; 

 and it is remarkable that many of the larva; of the first of these 

 groups resemble Entozoa in their parasitic habits. The head in larva! 

 of this kind, which are known as maggots, differs but little from the 

 segments of the body, the eyes in many cases not being developed, 

 and the mouth being furnished with a mere suctorial disc. In the 

 Lepidoptera and most of the Coleoptera however, the larva at the time 

 of its escape possesses the rudiments of the three pairs of thoracic 

 1 egs, although they are little else than simple claws, save in the carni- 

 vorous beetles ; whilst in addition to these, several of the abdominal 

 segments are furnished with fleshy tubercles or prolegs, generally to 

 the number of four or five pairs, which are peculiar to the larva state. 

 In such larvae, which are commonly designated as ' Caterpillars," wo 

 observe a remarkable equality in the different segments both as to 

 size, form, and plan of construction, which reminds us of the Annelida. 

 The alimentary canal occupies nearly the whole of the cavity of the 

 body, and passes without flexure from one end of it to the other. The 

 compartments of the dorsal vessel, the respiratory organs, the nervous 

 centres, and the muscular bands, are repeated with great regularity, 

 and there is as yet no distinction between the thoracic and abdominal 

 portions of the trunk. The head however is usually protected by a 

 horny covering, and is provided with simple or clustered eyes like those 

 of the higher Annelida and Myriapoda, and the mouth is furnished 

 with powerful cutting jaws for the division of the food, which is 



Fig. 3S. 



Fig. 3S, a Coleopterous Larva (Trlephonu). Fig. 30, Larva of an Ilcmip- 

 terous Insect (/.flat). Fig. 37, Larva of a Dipterous Insect (Calez). Fig. 38, 

 Larva of a Lepidopterous Insect (Sphinx). 



usually vegetable in its nature. In the Orthopterous and Hemipte- 

 rous orders however, these stages of development are passed through 

 within the egg, and as the young insect does not emerge thence until 

 HAT. HtBT. DIV. VOL. III. 



it has attained a higher grade, in which it presents a close resemblance 

 to its parents in almost every particular save the want of wings, it 

 cannot be regarded as having the characteristics of a real larva. This 

 is the case too with some of the Coleoptera, in which we find a con- 

 siderable variety as regards the stage of development at which the 

 embryo quits the ovum. In the true larva condition, the whole 

 energy seems concentrated upon the nutritive functions ; the quantity 

 of food consumed is enormous, and the increase in the bulk of the 

 body is very rapid. During this rapid growth the caterpillar throws 

 off and renews its epidermis several times, but the larvae of the 

 ffymenoptera and IHptera do not undergo this exuviation until they 

 pass into the pupa state, their integument being soft enough to yield 

 to the distension from within. 



The principal variations in the larvae of insects have been arranged 

 in the following tabular form by Messrs. Kirby and Spence (' Intro- 

 duction to Entomology," vol. iii.) : 



I. Larvao without legs. 



L With a corneous head of determinate shape. 



Coleopterous and Hymenopterous Apods Culicida?. Some 



Tipularice, &c., among the Diptera. 

 ii. With a membranous head of determinate shape. 

 (Muicidce, Syrphidcc, and other Diptera.) 



II. Larvao with legs. 



L With legs only and without an anal proleg. 

 (Neuroptera and many Coleoptera.) 



1. Joints short and conical (Elater, Cerambycidce.) 



2. Joints long and subfiliform. (Stap/tylinus, Coccinella, 



Cicindela, &c.) 

 ii. Prolegs only. (Many Tipularite, and some subcutaneous 



Lepidopterous larvae, &c.) 

 iii. Both legs and prolegs. (Lepidoptera, Serrifera, and some 



Coleoptera.) 



1. Without claws. (Serrifera, &c.) 



2. With claws. (Lepidoptera, &c.) 



In the Pupa state insects exhibit two principal modifications : those 

 which in general form resemble their larvae, and those which are 

 wholly unlike their larvse. 



Fig. 40. 



Fig. 41. 



Fig. 39, Pupa of a Lepidopterous Insect (Sphinx). Fig. 40, Pupa of a Butter- 

 fly. Fig. 41, Pupa of a Beetle. 



The former of these divisions Burmeistcr subdivides into incomplete 

 pupae and sub-incomplete. Incomplete pupa; are those without alary 

 appendages : to these belong the Lice and Bugs (Pediculua and Cimex 

 lectulariue). Sub-incomplete pupa) are those which possess rudiments 

 of wings. These comprise all the pupae of the winged genera of the 

 orders tfemiptera and Ort/ioptera. 



In the second division, comprising those insects iu which there is a 

 complete metamorphosis, the pupa, with very few exceptions, is 

 inactive, and does not take the least nourishment. They are divided 

 into Pupa Coarctata, or those which are covered with a case in which 

 no traces of the future insect can be perceived, as in Muaca and many 

 other Dipterous Insects; Pupa Obtecta, where the thorax and abdomen 

 are distinct and inclosed in a horny case, as in the Lepidoptera ; and 

 Pupa Incompleta, where the parts are covered by a membrane, but 

 distinct, as in the order Ifymenoptera and some Dipterous Insects. 



In the Coleoptera, Lepidoptera, ffymenoptera, Diptera, and some 

 Neuroptera, the pupa state is one of complete inactivity as regards 

 all manifestations of animal life; although the formative processes 

 are carried on with extraordinary energy. The imperfect larvae of 

 these orders are truly embryonic iu their condition ; and the processes 

 of development which were commenced in the egg, and which wero 

 then only carried far enough to enable the larvae to come forth and 

 obtain their own nutriment, are now continued at the expense of the 

 food which they have collected and stored up within their bodies ; so 

 that the passage into the pupa state in such cases may be fairly 

 likened to a re-entrance into the egg. The pupa is inclosed iu the last 

 skin exuviated by the larva, which, instead of being thrown off, dries 



