MICROSCOPIC OBJECTS. 78 



Fig. 155. Foot of a Fly (Scatophaga stercoraria), x80. 



At the extreme end of the tarsus, or foot, in most flies, is 

 found a small pad or cushion called the pulvillus. It is 

 generally placed between two claws, or hooks (see drawing), 

 and is supposed to enable the Fly, by its suctional action, to 

 walk on smooth surfaces. The pulvillus is often covered with 

 a number of disk- like hairs, and is also occasionally accom- 

 panied by a strong hair-like appendage. This must also 

 greatly assist the Fly in walking over any smooth surface, as 

 it would take advantage of any irregularity upon it. After 

 the pad has been placed in its naturally extended position, it 

 must be dried under very strong pressure, and mounted in 

 balsam as usual. 



Fig. 156. Eye of the Common House-Fly (Musca domestica), 



x200. 



These facets are much smaller than those figured in fig. 

 137. The eyes of insects, as mentioned in the description of 

 fig. 126, consist of a number of simple eyes, or facets, joined 

 together at their angles, so as to form an oval convex surface 

 protruding from the head, one on each side. In each single 

 eye of the House- Fly there are about 4000 facets ; and some 

 insects, such as the Dragonflies &c., have even more than 

 this. 



After having macerated the eye in liquid potassa for about 

 six hours, so as to clean it from blood &c., it must be well 

 washed, a slit cut in it, then dried between writing-paper 

 without much pressure, and mounted in a shallow dry trans- 

 parent cell made of cardboard; or the entire eye may be 

 mounted in a dry opaque cell without any previous prepara- 

 tion. 



