PROCURING OBJECTS. 59 



The external form of the eye may be seen in situ in all in- 

 sects when viewed as opaque objects, but the layer of lenses 

 requires the aid of maceration and dissection to free them 

 from a considerable amount of pigment. They may then be 

 mounted dry, in fluid, or in balsam. If required to be flat, 

 they must be made so by pressure while soft, otherwise they 

 are liable to split. 



If the eye of a fly, or other insect, properly prepared by 

 mounting in balsam, be held near the eye of an observer who 

 looks through it at a distant candle, &c., the interference of 

 light in the minute lenses will cause a number of images to be 

 perceived, tinged with beautiful colors. 



The eyes of spiders are single. They have from four to 

 twelve, variously arranged. Some insects have also single 

 eyes in addition to the compound eyes before noticed. 



Feet. The structure of the feet of those insects which sup- 

 port themselves on polished surfaces, and against the force of 

 gravity, is very remarkable, and it is doubtful if it be yet per- 

 fectly understood. Some suppose them to act as suction-pads, 

 others that they secrete a viscid fluid by means of which they 

 stick with sumcient force to enable them to walk. The latter 

 theory is rendered most probable by microscopic researches. 



The number of pads on each foot is variable. 



The anterior and middle pairs of feet of the male Dytiscus 

 are furnished with curious disc or cup-shaped appendages on 

 the inside of the leg. They may be viewed as opaque and in 

 balsam. 



Hairs of Insects, &c., may be mounted dry, in fluid, or in 

 balsam. In some spiders the hairs are branched; in the larvae 

 of many insects they are covered with spines, as the hairs of 

 caterpillars, &c. ; and in the Crustacea they are provided with 

 spines, or plumed like a feather. The hairs and scales of in- 

 sects will be further treated of in the chapter on Test Objects. 



