290 ANIMAL PHYSIOLOGY. [LESSON 90. 



eyes, grouped together on the two sides of the head. The eyes of 

 Scolopendra (Centipede) consist each of a group of about twenty- 

 three small, distinct eyes, approximated and placed in lineal rows, and 

 the aggregated eyes of lulus are also composed of several rows. 



1303. In Insects the eyes, like most other organs of the body, at- 

 tain a high degree of development ; moreover, two kinds of visual 

 organs are found in many of them, one series of eyes being adapted 

 for long, and the other for short sight. The great Dorr-beetle, or 

 May-bug, as the Melolontha vulgaris (Cockchafer) is commonly 

 called, is a familiar example of an Insect endowed with very short 

 vision. Persons walking in the fields, especially at twilight at the 

 latter end of the month of May, or during June, will find these In- 

 sects constantly striking them in the face, and flying against other 

 parts of their body, with so much force that the creature frequently 

 falls, as if stunned by the concussion. 



1304. The facetted, or compound eye, so conspicuous in all the 

 perfect insects, appears to have but a short focal range, and even this 

 differs to a large extent, as may be seen by the superior and remark- 

 able convexity of these eyes, in many of them. 



1305. Those insects which feed on the juices of plants, or ani- 

 mals, require very short vision, as their food lies at their feet, and 

 hence their eyes are singularly convex. 



1306. Amongst the predaceous beetles, longer vision is required, 

 and the convexity of the compound eyes is greatly reduced. 



1307. The Bees, on the other hand, need telescopic vision to 

 guide them in their long flights in search of honey and wax ; but, 

 when engaged in the act of collecting, or in making, and hermetically 

 sealing their beautiful cells, near vision becomes necessary, and hence 

 much convexity of their compound eyes. 



1308. The telescopic form of eye, when present, is in the form of 

 two or three single, distinct eyes, of larger size than any of the facets 

 of the compound eye, and placed in the best possible position for the 

 exercise of their function on the vertex (crown of the head). Cat- 

 erpillars have only a variable number of these single eyes, grouped 

 together, however, as a common mass, with but a moderate interval 

 of space between them. 



1309. Much disagreement prevails in regard to the true structure 

 of the Insect eye, and of the three authorities who have chiefly ex- 

 amined it, no two of them hold the same opinion. These authorities 

 are, Marcel de Serres, Straus-Durckheim, and J. Miiller. 



1310. According to Straus, the epidermis continues over the ex- 



