Variations 135 



a mate, and lay its eggs in a suitable position. The larva, 

 on the contrary, is an animal of very simple mode of life, 

 feeding upon dead vegetable matter at the bottom of dark 

 and slow streams. The abundance of its food, and the 

 ease with which it can be appropriated, have led in this, 

 as in many other cases, to some degree of degeneration 

 which is particularly apparent in the larval limbs and 

 head ^ We have already pointed out (p. 30) that the 

 brain must be removed to the prothorax in an insect 

 whose imaginal head develops in the prothorax, and that 

 this shifting of the brain would naturally lead to further 

 reduction of the larval head. 



We find that within the family Chironomidae there variations 

 are considerable variations in the mode of formation of 

 the imaginal head. Thus in a large Chironomus-larva, 

 of which we have not been able to procure the flj^, the 

 compound eyes are restricted, even in a la.te stage of 

 formation, to the larval head. It is noteworthy that 

 in this larva the head is much larger than in C. dorsalis. 

 In some Tanypus- larvae the same thing has been found. 

 In other Chironomus-larvae the compound eyes, just 

 before pupation, lie half in and half out of the larval 

 head, and here too the head is larger than in C. dorsalis. 



While the testes, sperm-ducts, and their contents are Deveiop- 



, . • J. • ment of re- 



undergomg development, a paired ventral invagination productive 

 forms in the last abdominal segment of the larva. This *"'^**°'- 

 soon becomes double and lengthens greatly, bending first 

 forward, then backward, and lastly again forward. From 

 it are derived the paired ejaculatory ducts (ducts of 

 Herold). They are the ectadenes (i. e. ectodermal glands) 

 of Escherich, who distinguishes glandular mesodermal 

 outgrowths, e. g. outgrowths from the sperm-ducts, by the 



' W^e have to thank the Linnean Society for permission to extract part 

 of our paper of 1892, and to copy several figures from the illustrated 

 plates. 



