428 Reeent Literature. [July, 
formed morphological rudiments, the number and the morphological im- 
portance of the organs transformed from the organs of the larva, and of 
those which pass, without change, into the organs of the imago. 
“I deem it proper to examine here the question of the morphological 
importance of the imaginal discs of insectsin general. The data respecting 
their embryology and comparative anatomy render it very probable that 
the thoracic imaginal discs, hidden in the body of Muscidae, the thoracic 
imaginal dises placed immediately on the skin of Corethra, Miastor, and 
the Hymenoptera, and the thoracic legs of the larve of Lepidoptera 
and Coleoptera, are homological formations, replacing each other in all - 
those groups. In other words, and more explicitly, I believe that the 
thoracic imaginal discs of the Hymenoptera, Muscide, Corethra, and 
Miastor are nothing but reduced ambulatory legs, which in other insects 
(Lepidoptera and beetles) are used as organs of progression, but in the 
above-mentioned groups (Muscide, etc.), have lost their physiological 
value, and have preserved in the history of their development a mere 
record of that value. This view may be sustained by the following 
scientifically pregnant facts: (1.) All insects, the larvae of which possess, 
in their thoracic segments, the so-called imaginal discs, do not have any 
rudiments of legs on the same segments during the period of their em- 
bryonal development; in other words, the imaginal discs take the place 
of the legs, which, in other insects, appear much earlier, in the same 
places, during the period of the embryonal development. (2.) In insects, 
the larve of which possess thoracic legs, these latter are transformed 
into the legs of the imago, in such a manner that the final segmentation 
of the joints of the leg of the imago appears more or less sudden and 
simultaneous, in consequence of the segmentation of the corresponding 
leg of the larva, which has been very much drawn out in length. On 
the contrary, those insects, the larve of which, instead of thoracic, am- 
bulatory legs, have imaginal discs, show, before the appearance of the 
final segmentation of the leg of the imago, a stage of a provisional seg- 
mentation of the leg in the developing imago. Thus, the segments of 
the leg of the imago of Muscide, Hymenoptera, etc., do not all appear 
simultaneously, but gradually, first one, then two, three, etc. This pro- 
_Visional segmentation of the leg, growing out of the imaginal disc, must 
be considered, probably, as the expression of the ultimate segmentation 
of the leg which it formerly possessed ; or, in other words, the provisional 
segments of the leg, developing from the-imaginal disc, remind us of the 
permanent segments of the larval legs of Lepidoptera, beetles, Hie 
which, in these latter, are used as temporary, provisional, locomotive p a 
gans. (3.) I believe that great morphological importance must ke 
tached to the fact that during the development of the imaginal disc of the | 
Muscide, the H noptera, Corethra, and Miastor, the provisional cav- 
ity in the disc, which has no ultimate meaning, appears first of 2 — 
e scientific meaning of this provisional cavity, as well as of its out- 
