668 



TEXT-BOOK OF ENTOMOLOGY 



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a. Development of the outer body-form 



The form of the imago is completely marked out in the pupa, so 

 that the transition from the pupa to the imago is comparatively 

 slight and only depends on the modification and development of the 

 parts already present. 



In most cases the modification in question consists of the changes 

 occurring during the passage from the larval form to the imago, the 

 reformation of parts already present being most marked, while the 

 new rudiments only participate in a limited way in the process. 

 Thus, for example, the head of the caterpillar together with the 

 antennae and mouth-parts, also the thoracic limbs, pass directly and 

 unchanged from the larva into the pupa. The compound eyes and 



the wings are, however, 

 new formations, the 

 latter arising from im- 

 aginal buds. The same 

 is the case with many 

 other Heteroinetabola, 

 where the passage of the 

 larva into the pupa in 



FIG. 622. Anterior part of young larva of Simuliwm general IS due to a trans- 

 "icea, showing the thoracic imaginal buds: p, prothoracic -frvT-mofim-i n-F -r>ovfo ol 

 bud (only one not embryonic) ; w, w', fore and hind wing- 

 buds ; I, I', I", leg-buds ; n, nervous system ; br, brain ; e, eye ; vpo/1-.r n r p p n t TVip 

 sd, salivary duct ; p, prothoracic foot. After Weismann. * U J 



changes in the brain, 



the fusion of certain ganglia of the ventral nervous cord, the 

 changes in the abdomen, involving the reduction in the number of 

 segments and the remodelling of the end of the body, and the for- 

 mation of the ovipositor or sting, and in the higher Hymenoptera 

 the transfer of the 1st abdominal segment to the thorax, and the 

 origin of the genital armature, all these should here be taken 

 into account. 



It should be observed that in every case where the larvae are foot- 

 less, as in Diptera, all the Hymenoptera except the phytophagous 

 ones and certain coleopterous larvae, the limbs of the imago stage 

 are, in the earliest stages, indicated as new structures in the form of 

 imaginal buds. 



Formation of the imago in Corethra. Corethra may serve as 

 an example of such a relative!} 7 simple metamorphosis. Its larva 

 belongs to the group of eucephalous dipterous larvae. The head of 

 the perfect insect is already indicated in the larva, and its parts, 

 with certain modifications, pass directly into the pupa. The com- 



