£ntornology of the Illinois River. 161 



other two sides of wire gauze. The upper rim is all of 

 wood and supports a close fitting glass cover in a wooden 

 frame, overhangiug outwardly, so that the whole in- 

 terior is visible through the glass. (See Plate XV.) These 

 cages are so placed in the lake or river as to be about half 

 full of water, thus maintaining the quality of water and 

 the temperature natural to life occurring there. 



CHARACTERS USED IN THE KEY TO ORDERS. 



A typical insect larva or pupa has about a dozen 

 usually well-marked divisions. The first is the head, the 

 next three constitute the thorax, and the remainder the 

 abdomen. In the stratiomyiids the thoracic segments 

 are closely like those of the abdomen, but there is usually 

 a noticeable difference. In most larvae the thoracic seg- 

 ments are readily distinguished by the pair of jointed 

 feet on each, and in the pupa or older nymph, by a con- 

 spicuous backward extension ftora each side of the second 

 thoracic segment, more or less covering a smaller pair 

 of similar extensions on the third, these being the two 

 pairs of wing-pads. The second pair is rudimentary in 

 the Diptera. The distinction between the larvae of 

 groups A and B is not applicable to v6ry young indi- 

 viduals, but one will easily learn to know those of the 

 first group, as they closely resemble the older stages, 

 and the inter-resemblances are quite marked in each of 

 its subdivisions, which are not numerous. 



The spring-tails are minute wingless insects, many 

 of which frequent wet shores, often hopping on the 

 surface, but rarely discovered beneath it. The next three 

 oi-ders take up air from the water, usually by flat mem- 

 branous gills upon the thorax, or the sides or the end 

 of the abdomen, or within the end of the alimentary 

 canal. The terminal setae are long antenna-like tails. 

 The true bugs breathe air directly, those which swim 

 in water coming to the surface for it. 



In group B the abdomen often bears beneath fleshy 

 jointless prominences, used as feet and called false feet. 

 —11 



