Fig. 1 : 4. Diagram of archetype wing venation with veins named 

 according to Comstock-Needham system. A, anal veins; C, costa; 

 Cu, cubitus; M, media; MA, media anterior; MP,, media posterior; 



Pc, precosta; Sc, subcosta; R, radius; R„, radial sector (Snodgrass, 



1935). 



In most insects there is considerable difference 

 between the first instar and the adult. The process 

 of change which occurs between these stages is 

 called metamorphosis. The types of insect metamor- 

 phosis can be classified in three principal categories 

 according to the degree of change which takes place. 



1. Ametabolous metamorphosis. — Some insects, 

 such as the Collembola, emerge from the egg resem- 

 bling miniature adults, and change of form from instar 

 to instar is very slight, being limited primarily to 

 increase in size and development of functional repro- 

 ductive organs. Such insects are called Ametabola, 

 and others exhibiting distinct metamorphosis are 

 termed Metabola. The latter may be further divided 

 into the Hemimetabola and the Holometabola. 



2. Hemimetabolous metamorphosis. — Insects in this 

 category undergo simple metamorphosis, often termed 

 direct or incomplete. The immature stages greatly 

 resemble the imago in general structure, body form, 

 and mode of life and are called nymphs. Change of 

 form consists primarily of gradual development of 

 wings as external pads and of the reproductive organs. 

 The Hemiptera typify this type of metamorphosis. 



In the Plecoptera, Ephemeroptera, and Odonata the 

 nymphs are aquatic and the adults aerial. Since the 

 nymphs of these three orders possess special respir- 

 atory organs and other adaptive modifications for life 

 in the water, the final transformation into the adult 

 involves more profound morphological change than 

 among other Hemimetabola. Plecoptera nymphs have 

 only relatively minor structural adaptations for aquatic 

 life; the most conspicuous are the gills possessed 

 by some species, the vestiges of which are carried 

 over to the adults. In the Ephemeroptera and Odonata 

 the nymphs have well-developed lateral or anal gills 

 and other special structures not carried over to the 

 adult. They are so entirely unlike the adult in general 

 appearance that the relationship is not at all self- 

 apparent. The nymphs of these orders are called 

 naiads by some writers. 



3. Holometabolous metamorphosis. — In this type of 

 development, also called complex or indirect, there 

 are three distinct growth forms: the larva or feeding 

 and growing stage, the pupa or resting and trans- 



71 

 Bentinck: Structure and Classification 



formation sta^e, and the adult. The larva i- quite 

 different in appearance from the adult and frequently 

 bears no resemblance whatsoever to it. It is often 

 wormlike, without legs, and the eyee and antennae 

 may be small or aborted. Thoy do not have external 

 wing pads as do hemimetabolous nymphs, but the 



wings develop as internal pads and arc called imag* 

 inal buds or histoblasts. When the larva transforms 

 to a pupa these are everted and are then external 

 wing pads. Frequently the larva spins a cocoon, 

 builds a protective case, or excavates a chamber in 

 which to pass the pupal stadium. 



Among the many types of larvae there are a few 

 so common as to merit distinctive names. The more 

 important of these encountered among the aquatic 

 insects are the following: 



Campodeiform (fig. 13:10n). — In this type the man- 

 dibles are well developed, the body is elongate, 

 flattened, with three pairs of thoracic legs and usually 

 with cerci or caudal filaments. Typical examples are 

 larvae of Dytiscid beetles. 



Eruciform (fig. 11:1c).— Not common among aquatic 

 insects, this type is represented by larvae of Lepi- 

 doptera. The body is cylindrical, the thoracic legs 

 short, and the abdomen is furnished with prolegs on 

 some segments. 



Vermiform (fig. 14:63d, e, k, m). — Larvae which are 

 more or less wormlike in form and without legs are 

 termed vermiform or apodous. Characteristic of this 

 type are the larvae of most Diptera,many Hymenoptera, 

 and the weevils among the Coleoptera. 



The pupa is a nonfeeding, quiescent stage during 

 which transformation from larva to adult occurs. At 

 this stage the wings and appendages of the future 

 adult become evident externally. There are three 

 principal types of pupae. 



Exarate (fig. 13:39). — In this type all appendages 

 are free of any secondary attachments to the body. 

 Pupae of Trichoptera and most Coleoptera are char- 

 acteristic of this type. 



Obtect. — Obtect pupae have the appendages tightly 

 appressed or fused to the body. Most Lepidoptera 

 are of this type. 



Coarctate (fig. 14:63c, h, j, s). — In the Diptera Cy- 

 clorrhapha, the last larval exuvium persists and 

 becomes transformed into a hardened barrellike cap- 

 sule or puparium that protects the exarate pupa within. 

 A pupa so encased is said to be coarctate. 



The adult is the final instar and possesses func- 

 tional reproductive organs and, in most cases, wings. 

 Its primary function is reproduction. 



Classification may be defined as the systematic 

 arrangement of organisms in groups or categories 

 according to some definite plan or sequence. The 

 principal categories used herein are, in order of 

 descending rank: class, order, family, genus, and 

 species. The criteria by which these categories are 

 defined may be purely arbitrary, in which case the 

 resulting classification would be an artificial one; 

 or an attempt may be made to select criteria which 

 elucidate the natural relationships and evolutionary 

 history of the various forms involved. In the latter 

 case the desired result would be a natural or phylo- 



