454 THE INVERTEBRATA 



to the heart rudiments, and on their outer and inner borders to the 

 muscles of body wall and gut respectively. The lower border of each 

 somite breaks down to form fat-body. In so doing the coelomic 

 cavity disappears, and, minute as it always was, becomes continuous 

 with those spaces arising by separation of the germ layers from each 

 other, viz. the haemocoele. This latter as in all Arthropods consti- 

 tutes the main body cavity (Fig. 316 E). 



Metamorphosis. Insects, like all other arthropods, attain their 

 maximum size by undergoing a succession of moults or ecdyses. The 

 number of moults which an insect passes through is fairly constant 

 for the species, and the form assumed by the animal between any 

 two ecdyses is termed an instar. The animal's existence is thereby 

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

 the simplest and most generalized 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 re- 

 productive system. Where the adult is primitively wingless, as in 

 silver fish and springtails (Fig. 323), 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 Ametabola. 



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. 341). The degree of metamorphosis varies consider- 

 ably, irrespective of wings, in winged insects according as the young 

 stages resemble their adults or not. A growth stage of a cockroach, 

 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 re- 

 semblance to the final instar with its wings, elaborate body form and 

 mouth parts (Fig. 349). 



Metabolous insects, those passing through a distinct metamor- 

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

 Heterometabola, 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- 



