i84 



Marvels of Insect Life. 



which connect with the brain. The great convexity of these simple eyes gives them 

 a very short focus, and it is very probable that they are of use more for enabling the 

 Insect to judge of the intensity of the light rather than for purposes of distinct vision. 

 At any rate, Insects that have to rely upon them must be short-sighted. Insects 

 such as bees, wasps, and their allies that have to pass a good deal of time in the 

 darkness of their nests, have them well developed, and the same thing may be said 

 of the night-flying moths, which, however, have only two simple eyes. \\'ith the 

 exception of one genus, none of the butterflies have simple eyes. It thus appears 

 that these simple eyes are chiefly useful in the dark. The structure shows that they 

 can only be used for very near objects, probably within an inch. 



The compound eye, such as we see standing out prominently on each side 

 of the head of butterflies, moths, flies, dragon-flies, etc., may be considered as an 

 immense assembly of single eyes packed closely together, the separate lenses being 

 called facets. The number of these facets in a compound eye varies greatly. 



A 



ti) l.ii]~)li()(k, a l^)razil 



pccics of beetle ^ has only seven facets of unequal 

 size ; the silver-fish Insect has twelve, 

 the ant fifty, the house-flv four thousand, 

 the death's-head hawk-moth twelve 

 thousand, the swallow-tail butterfly seven- 

 teen thousand, a large dragon-fly - twenty 

 thousand, the convolvulus hawk-moth 

 twenty-seven thousand. The size of the 

 lacets appear to vary considerably, but 

 is generally proportionate to the size of 

 the Insect — thus the facets of an ant would 

 be much smaller than those of a dragon- 

 fly — but there seems to be a minim imi, 

 for the smallest Insects that have been 

 examined from this point of view have 

 facets not less than ._ioo«ith of an 

 inch across. 



These compound eyes are wonderful in their complex and delicate organization, 

 and compared with them the eyes of backboned animals are verv simple organs. 

 The external layer of the compound eye consists of the specialized cuticle, made 

 transparent and broken up into facets. Behind each facet is the cr\stalline lens 

 or cone, consisting of four or more cells of the lower cuticle. In the earwigs, most 

 bugs, daddy long-legs, and many beetles, the cone is not present. Behind the cone 

 is the rod formed of six filaments of chitin surrounded by nerve fibrils arising from 

 the optic nerve. Both cones and rods are buried in pigment, which prevents the 

 rays of light passing from one facet-eye to its neighbours. Below the rods is the 

 basal membrane which separates the instrumental part of the e\-e from the optic 

 tract, which perceives the image and transmits it to the brain. Tlie compound 

 eyes are very rarely found in Insects that have not reached tlunr final stage of 

 development; they occur, however, in the nymphs of dragon-flit's and may-flies, 

 and in the early stages of one of the gnats.^^ As to \\liat impression is transmittt'd 

 1 Lathridius. - .li.sclina. » Corethra. 



A Compound Eye. 



A portion of a compound eye of the great water-beetle is here 

 shown, magnified sixty-six times. It is believed that each of the 

 numerous lenses of which it is composed gives the Insect an image 

 of only a part of the object it is looking at. 



