INSECTS 317 



power, moreover, shows that this organ is composed of as many elon- 

 gated pyramid- shaped bodies as there are facets. Each of these pyra- 

 mids must accordingly be regarded as a single or separate eye, the 

 whole aggregate of which form the faceted or compound eye. The 

 same section will reveal the following further details of structure : The 

 outermost portion of each pyramid is formed of a hexagonal columnar 

 body (S), the outer convex surface of which forms the facet (F). Being 

 of glassy transparency, this portion allows the free entrance of light into 

 the interior of the pyramid. Behind it we find (in most insects) a cone 

 of crystalline transparency (K), which conducts the rays of light to the 

 posterior portion of the single visual element (I.A.). This latter portion 

 is highly sensitive to light impressions, and therefore comparable to the 

 retina of the vertebrate eye. Bays of light, which enter the eye 

 obliquely, and which, by penetrating into adjacent single eyes, would 

 interfere with distinct vision, are absorbed by a pigment which, surrounds 

 the crystalline cone, and the lower part of the sensitive (retinal) portion, 

 and are thus rendered ineffectual. 



How, then, does an insect see with this peculiar organ ? Laborious 

 researches have shown that only a portion of the image of the object 

 seen by the insect is formed in each single visual unit. The adjacent 

 unit sees another part of this image, and so on, so that the total image in 

 the eye of the insect is composed of a number of separate images, just as a 

 pavement or a picture in mosaic is composed of a number of separate stones. 

 We can now also understand the great size and convexity of the insect 

 eye : from whatever direction the light rays proceed they will neverthe- 

 less penetrate one or other of the numerous single eyes and impress 

 themselves on the animal's consciousness. (We may approach a house- 

 fly from whatever direction we please : the insect always notices it, 

 escapes, and is only caught with difficulty.) 



In the larvae the eyes are of much simpler structure, as are also the 

 so-called accessory eyes, ocelli or stemmata (explain these names), which 

 are found on the front portion of the head in some fully- developed insects 

 (e.g., the bee). 



(b) The antennce vary much in shape (give examples) and, like the 

 legs, are always jointed (why ?). They discharge the functions of tactile 

 organs (hence also called "feelers"), but also — as has been shown by 

 numerous experiments — act as organs of smell, and, in many species, of 

 hearing (for the auditory organs of the locust see under that species). 

 A large number of insects seem to be completely dumb. (For the palpi 

 see Section 3.) 



7. Metamorphosis. — The larva (see p. 310) and the perfect insect 

 differ from each other in structure to a greater or less degree, according 



