02 ON THE ANATOMY OF THE FLY. 



medium and their position in a triangle upon the forehead, 

 seem not only eminently adapted for the perception of very 

 near objects, but also for the nicest possible appreciation of their 

 distances from the insect. 



Beneath the cornea in the ocelli is a thick layer of nerve 

 cells, which probably receives the impression, and it is quite 

 likely that the adaption to distance depends upon the portion of 

 this layer upon which the picture falls. I see no reason why 

 the deeper cells should not receive an impression as well as the 

 more superficial, and the difference of the focus cannot be very 

 great. The whole internal surface of the ocellus is covered with 

 pigment of a bright orange red color, and the three ocelli are 

 connected with the cephalic nerve-centre by a single nerve. 



The antennae have the third joint remarkably dilated, but 

 the others are comparatively little developed, except the last, 

 which is very slender and covered with hairs. The third joint is 

 covered all over with minute openings, which communicate 

 with little transparent sacs or groups of sacs in the interior. The 

 whole joint is lined with orange red pigment cells, and filled 

 with a pulpy mass, which consists of the antennal nerve, minutely 

 divided, and bearing multitudes of very small nerve cells, 

 arranged like bunches of grapes at the extremity of its divisions- 

 I believe myself that this is the organ of smell, although I by 

 no means consider the antemiee of all insects are necessarily 

 olfactory organs. I think in many instances they are merely 

 feelers. Perhaps the beautiful feather-like aiitennee of male 

 moths are sexual ornaments, although they may have special olfac- 

 tory organs connected with them ; and possibly the laminated an- 

 tennae of many beetles, which consist of thin chitinous lamellae, 

 may be hygrometric, indicating the state of the atmosphere to 

 the insect. I have littlg doubt, however, in other insects, 

 as in the fly, especially when they are thick and club-shaped, 

 that they are olfactory, or rather partly olfactory organs. 



The halteres are most evidently modifications of the posterior 



