OF THE HEAD OP THE IMAGO OF CHIKONOMUS. 275 



Id Corethra, as Weismann has shown, the compound eye develops in immediate 

 proximity to the larval eye, but the imaginal antenna forms by an invagination, 

 reaching far hack into the larval head. 



In order of complexity of the invaginations which give rise to the head of the imago 

 we should arrange the Dipterous types already mentioned as follows : — 



1. Culex. Eelatively simple. Invaginations shallow. 



2. Corethra. Simulium. } T , v , 



> Intermediate. 



3. Chironomus, Ceratopogon. J 



4. Muscidce. Relatively complex. Invaginations deep, and apparently, but not 



really, unconnected with the epidermis. 



The development of the head of the fly of Chironomus appears therefore to furnish a 

 useful middle term between the Adiscota and the Discota of Weismann : that is, between 

 the types in which the parts of the head of the fly are developed in close relation to 

 those of the larva, and the types in which deep invaginations lead apparently to the 

 formation of similar new parts far within the body, the seeming independence of the 

 new parts being intensified by thorough-going histiolysis. Other Dipterous types 

 intermediate between Corethra and Chironomus, or even simpler than Corethra, yet 

 require description, and possibly types may be discovered intermediate between Chiro- 

 nomus and Iluscidce. It will also appear, when the subject is fully gone into, that insects 

 of other orders, e. g. Lepidoptera, present good examples of the origin of imaginal 

 organs by foldings-in of the larval epidermis *. 



Mr. Poultonf has pointed out that it is entirely erroneous to speak and think of the 

 various parts of the Lepidopterous pupa as mere cases for the corresponding j)arts of the 

 imago. " If we examine a section of a pupal antenna or leg (in Lepidoptera) we shall 

 find that there is no trace of the corresponding imaginal organ until shortly before the 

 emergence of the imago. In the numerous species with a long pupal period the 

 formation of imaginal appendages within those of the pupa is deferred until very late, 

 and then takes place rapidly in the lapse of a few weeks. This also strengthens the 

 conclusion that such pupal appendages are not mere cases for the parts of the imago, 

 inasmuch as these latter are only contained within them for a very small proportion of 

 the whole pupal period." 



It would appear from this passage and from what we have seen of the development of 

 the imago of Chironomus that there is a strong superficial contrast, as to the formation 

 of the imaginal organs, between Lepidoptera and Tipularian Diptera. Chironomus and 

 Corethra exhibit an early and protracted metamorphosis, which extends through the last 

 larval stage, as well as the relatively short pupal stage. Before the larval skin is shed the 

 compound eyes, the antennae, the wings, the legs, and reproductive organs, both external 

 and internal, are far advanced, and though not complete in all points (the corneal facets, 



* J. Dewitz has shortly described (Biol. Centralblatt, Bd. iii. 1883-84) the formation of theimaginal antenna of 

 Pieris Brassica by a process essentially the same as that described in Chironomus, though far simpler, 

 t " External Morphology of the Lepidopterous Pupa," Trans. Linn, goc, 2nd ser. Zool. vol. v. p. 188 (1890). 



tl* 



