GENERATION OF INSECTS. 433 



state. It seems to be contracted by a series of intus-susceptions ; 

 the abdominal part is slightly expanded, more definitely divided into 

 chambers, and better provided with valves ; the thoracic portion is 

 simplified, shrunk in diameter, and is more distinctly defined as an 

 aorta sent off from the heart. {Fig. 160.) 



The respiratory system undergoes still more remarkable modifica- 

 tions. The branchiae of the aquatic larvae either disappear or are 

 developed into wings : the long pneumatic tubes of those which, 

 living in water, breath air, shrink and disappear. The partial dilata- 

 tions of certain tracheae, to form reservoirs of air for diminishing the 

 specific gravity of the body, begin to be formed in the pupa state of 

 the flying insect. 



Herold has shown that germs of the generative organs exist in the 

 larvae of the Lepidoptera; the testes appear on each side as four 

 nucleated cells in a longitudinal series, which, by progressive coales- 

 cence longitudinally, by approximating transversely, and ultimately 

 uniting at the middle line, first form an eight-chambered, and after- 

 wards a spherical gland {Jig. 160, s). The ovaria, retaining their 

 primitive separate state, increase in length and assume the spiral dis- 

 position in the pupa state. 



The progressive changes which the nervous system of the Lepido- 

 pterous insect undergoes, in its metamorphoses from the larval into the 

 perfect state, have already been described (p. 366); and I need only now 

 remark that the general principle of those changes is like that which 

 governs the modifications of the muscular system, viz., a localisation 

 of special masses at particular parts for special purposes ; the result 

 of which is the departure from a common to a particular type of 

 arrangement. 



One of the most obvious and remarkable phenomena in the larval 

 life of an insect is the successive sheddings of the skin. The number 

 and frequency of the ecdyses vary in different species and relate to 

 two circumstances, viz., the rapidity of the growth of the body, and 

 the susceptibility or otherwise of the skin to be distended or to grow 

 with the increase of the body. 



The soft-skinned maggots of many flies, which acquire a vast in- 

 crease of size during their brief larval state, never moult until they 

 change into pupae, when the exuvium forms the pupa-case. In like 

 manner^ the soft -skinned apodal larvae of the Hymenoptera do not 

 moult until they have acquired their full size. The caterpillars of 

 the Lepidoptera moult at least three times, and some more frequently; 

 the Bomhyx villica, for example, from five to eight times, and the 

 tiger-moth {Arctia caja) ten times. 



With regard to the nature of the mutations and moults which 



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