3i6 



TEXT-BOOK OF ZOOLOGY 



the heart in its pure and oxygenated state. In insects the air, as it 

 were, goes in search of the blood, whereas in other animals the reverse 

 is the case. The muscular heart-tube merely keeps the blood in constant 

 motion. The blood reaches the different organs by interstices and lacunae 

 between the tissues, and in a similar manner it returns to the heart (see 

 p. 309 and illustration, p. 307). At the same time it bathes the intes- 

 tine, separates from it the nutritive fluids, and conveys ^ 

 these to all the organs of the body. 



6. Organs of Sense — (a) Eyes. — In contrast to the 

 eyes in vertebrates, those of insects are quite immov- 

 able. The head being at the same time also but slightly <*~ 



K. 



1%: 



Compound Insect Eye (Diagrammatic). 



N., Optic nerve ; L., a trachea. Both enter the eye by the sieve-like perforated floor seen below 

 on the right. On the left, and at the outer edge of the right half, the faceted surface is 

 seen, which is continued into the chitinous envelope of the body (Ch. ). On the right the 

 eye is laid open in order to show the collection of pyramidal "single eyes," of which it 

 is composed. At P.F. the pyramids are shown surrounded by pigment ; at P. without 

 pigment, in order to show the details of their structure. On the right are shown two of 

 the separate pyramids more strongly magnified — that on the left with its surrounding 

 pigment, that on the right without pigment. The letters are explained in the text. 



movable, the range of vision would be very limited (what disadvantage to 

 the animal would accrue from this ?) if these drawbacks were not counter- 

 balanced by the remarkably large size of the eyes, their strong convexity 

 and entirely different structure. If the eye of an insect is examined 

 through a magnifying lens, its surface is seen to be composed of a large 

 number of small, usually hexagonal or square areas, or facets. A thin 

 vertical section through the eye viewed under a strong magnifying 



