FSYCHE. 



IMAGINAL DISCS IN INSECTS. 



BY HENRY S. PRATT, PH. D., HAVERFORD COLLEGE, PA. 



(Annua! address of tht 



; president of the Cambrid.^e Kntoinological Club, S Ja 



Among insects wliicli have a complete 

 metamorphosis the organization of the 

 larva is in general similar to that of the 

 adult or imago, but the larva alwavs 

 lacks certain organs which the imago 

 possesses. The larval beetle or butter- 

 fly lacks wings, the larval bee has 

 neither wings nor legs which is also true 

 of the larval gnat, while the larval fly 

 has neither legs, wings nor head; and 

 in each case the imago possesses these 

 organs. As we ascend the scale of 

 development from the less highly to the 

 more highly specialized insects having 

 complete metamorphosis, we find a 

 constantly increasing sum of differences 

 between the larval and the imaginal 

 forms, and a correspondingly increas- 

 ing number of organs ^vhich are pos- 

 sessed by the imago and not by tlie 

 larva. This drawing apart is due, on 

 the one hand, to the higher specialization 

 of the imago and its consequent further 

 departure from the ancestral stage of its 

 ontogeny represented or suggested bv 

 the larva, but also hirgely to the retro- 

 gressive development of the larva itself. 

 In the highest insects, where the imago 

 is a highly specialized animal capable 

 of living only in a certain restricted 

 environment, tht.- larva is perhaps as 



higiily specialized as is the imago: 

 its environment is as sharply restricted 

 and its structure departs as far from 

 the phyletic type or stage it represents 

 as is the case with the imago. 



In the coleoptera, to consider first 

 one of the less highly specialized groups 

 of holometabolic insects, the environ- 

 ments of the larva and of the imago are 

 usually quite similar, or perhaps they 

 are exactly the same ; the organs of the 

 two forms are correspondingly similar, 

 and the transformations which must be 

 accomplished on the body of the larva 

 to prodfice the imago are but slight. 

 The imago differs from the larva prin- 

 cipally in that it has acquired wings, 

 elytra, compound eyes, and external re- 

 productive organs, but all the larval 

 organs with the exception of the mid- 

 gut become imaginal ones without great 

 change. The midgut in all holometabo- 

 lic insects undergoes a complete trans- 

 formation during the metamorphosis.* 



In the lepidoptera, to come to a 

 somewhat more highly specialized 

 group, the larval and imaginal environ- 

 ments are apparently widely different 



* Kowalevsky, A. Beitrgge zur Kenntnis der nacheni- 

 bryonalen Eutwicklung der Musciden. Zeit. f, wiss. Zool. 



45; 3. iSS;, p. 565. 



