490 COMPARATIVE ANATOMY CHAP. 



In a few Thysanura (Machilis, Lepisma, and especially lapyx) the ovarial tubes 

 (5-7 on each side) are placed in the abdomen in more or less strict segmental arrange- 

 ment. Each independently enters one of the two oviducts, which run through the 

 abdomen as straight canals. The two oviducts open externally by a short unpaired 

 terminal piece ; this common piece is said to be wanting in Machilis, only the outer 

 aperture of the two oviducts being in this case common to both. In Campodea and 

 the Collembola the ovaries and testes on each side are simple tubes. 



XII. Dimorphism Polymorphism. 



In all insects the males and females differ, not only in the arrangement of their 

 sexual organs, but also in various details of their outer organisation. This sexual 

 dimorphism is in some cases very remarkable, and is generally connected with the 

 absence of wings in the female, e.g. in the scale insects (Coccidce), in the luminous 

 Goleoptera (Lampyridce), and in a few Bombycina (Psyche, Orgyia). In the parasitic 

 Strepsiptera the females are legless, wingless, eyeless, and feelerless, and are thus 

 like maggots. They are viviparous, and remain, as long as they live, enclosed in 

 their pupal envelopes, inside the host, i.e. in the abdomen of various Hymenoptera. 



In colonial insects polymorphism arises in consequence of division of labour 

 between the individuals of the colony. In many colonial Hymenoptera (bees and 

 ants) only a few of the females (queens) are sexually mature and capable of re- 

 production. The great majority of the other females (workers) have reduced sexual 

 organs, and, among the ants, are wingless. Among the ants different forms of 

 workers may appear (soldiers and workers proper). In the colonial Termites, and also 

 among the Corrodentia, there are, besides the winged reproductive males and females, 

 unwinged males and females with rudimentary sexual organs ; these are again 

 divided into castes (workers, fighters), and vary in form accordingly. 



XIII. Development and Life-History. 

 A. The Metamorphoses of Insects. 



The greater part of the developmental processes, by means of which the adult 

 insect is produced from the fertilised egg, take place within the egg -envelope. 

 The time passed within the egg is the period of embryonic development. 

 The organism which escapes from the egg-envelope, or in other words, is hatched 

 from the egg, is always already very highly developed, and, apart from the 

 fact that it has no wings, .no mature sexual organs, and no compound eyes, 'is 

 provided with all the typical insect organs in functional condition. It possesses a 

 completely segmented body, antennae, mouth parts, thoracic limbs, developed nervous, 

 digestive, and tracheal systems, the dorsal vessel, the body musculature, etc. It 

 moves and feeds itself independently. It is called a larva. The larvae of the insects 

 are thus, when hatched, far more highly developed than the larvae of most other 

 Invertebrate. 



The changes which an insect undergoes before it becomes an adult sexually 

 mature animal (imago) are extraordinarily various, and are conditioned by a whole 

 series of co-operating factors, among which the most important are : (1) the degree 

 of deviation of the imaginal form from the racial form ; (2) the different modes of 

 life and of places of habitation of the larvae and imagines. 



I. The Apterygota (Thysanura and Collembola) are considered to have retained 

 their original wingless condition, and in other ways also appear to stand nearest to 



