INSECTA 399 



two ecdyses is termed an instar. The animars existence is thereby 

 made up of a succession of instars, the final one being the aduh. 

 In the simplest and most generahzed insects the several instars are 

 very similar to one another and only differ from their appropriate 

 adults in the absence of wings and the incomplete development of 

 the reproductive system. Where the adult is primitively wingless, as 

 in silver fish and sp ringtails (Fig. 304), the change from young to 

 adult is so slight as to be ignored, and metamorphosis, involving only 

 a development of the reproductive system, is conveniently regarded 

 as being absent. The insect orders falling in this category are grouped 

 under the heading Ametahola. 



In winged insects, however, the winged adult is in sharp contrast 

 to the wingless young stage. Such forms are said to undergo a meta- 

 morphosis (Fig. 320). The degree of metamorphosis varies con- 

 siderably, irrespective of wings, in winged insects according as the 

 young stages resemble their adults or not. A growth stage of a cock- 

 roach, for instance, possesses the general appearance of the adult. 

 On the other hand the young stage of a housefly is a grub and has 

 no resemblance to the final instar with its wings, elaborate body form 

 and mouth parts (Fig. 326). 



Metabolous insects, those passing through a distinct metamor- 

 phosis, are therefore further divided into two subclasses, (i) the 

 Hetewmetabola, e.g. the cockroach, and (ii) the Holometabola, e.g. 

 the fly. A classification of insects based on degree of metamorphosis 

 is therefore possible and such a basis for classification is used in all 

 modern systems. 



The orders composing the Heterometabola are the Orthoptera, 

 Dermaptera, Hemiptera, Isoptera, Embioptera, Psocoptera, Ano- 

 plura, Thysanoptera, Plecoptera, Ephemeroptera, Odonata, Mallo- 

 phaga, the last three orders being sometimes classed as Hemimetabola 

 owing to the young stages being aquatic and distinguished from the 

 adults by the possession of features adapting them to life in water. 

 The young stages of all the Heterometabola, however, strongly re- 

 semble their adults in body form, type of mouth parts, and the 

 possession of compound eyes, and are known as nymphs (Fig. 299). 



The orders composing the Holometabola are the Neuroptera, 

 Mecoptera, Trichoptera, Lepidoptera, Coleoptera, Strepsiptera, 

 Hymenoptera, Diptera, and Aphaniptera. The young stages of these 

 are known as larvae and differ from their adults in body form, mouth 

 parts, and the absence of compound eyes. So great- is the difference 

 between the larva and the adult that an instar known as the pupa 

 has been specialized to bridge the gulf (Fig. 320). This stage, one of 

 apparent rest, is actually one of great physiological and developmental 

 activity, and it is here that many larval tissues, e.g. the muscles and 



