14 



APPLIED ENTOMOLOGY 



There is usually a point or tip called the apex, somewhere along the 

 margin of the wing, though frequently the outline is so rounded that the 

 exact apex is uncertain. The front margin of the wing from where it 

 joins the body to where the edge begins to turn backward (in an extended 

 wing) is called the costa. 



Wings are entirely absent in some groups of insects, and it is probable 

 that these are the direct descendants of the earliest forms, before wings 

 were developed. In other cases where they are absent this is associated 

 with a parasitic life where wings might be a distinct disadvantage, or 

 with peculiar habits which would render them useless or even inconvenient, 

 and in such cases they appear gradually to have become lost. In the 

 flies the hinder pair is modified, forming small structures not wing-like, 

 called halteres. 



The abdomen does not usually show great differences in its seg- 

 ments except those near the hinder end, which may be modified for 

 various purposes. Generally a dorsal plate and a ventral plate are the 

 only two skeletal plates evident in a segment. Small openings, usually 

 a pair in each, or at least in most, of the segments are the openings of the 

 breathing organs, and these also occur on some of the thoracic segments 

 where they are ordinarily less noticeable than on the alidomen. 



Fig. 21. — Larva of C'ecTopia Moth showing abdominal legs. Two-thirds natural size. 



{Original.) 



Legs are very rarely present on the al^domen in adult insects, but are 

 often found in the earlier stages '(Fig. 21), At the end of the abdomen 

 in the females of those insects which lay their eggs within objects, is a 

 combination of pieces known as an ovipositor. It usually consists of 

 about three pairs of parts, long or short, slender or stout as the case may 

 be, for the purpose of making a hole or sawing a slit in the object in which 

 the eggs are placed and in guiding the eggs into the hole thus made. In 

 one group which has apparently changed its habits and no longer needs 

 to make holes for egg laying, the ovipositor being unnecessary for this 

 purpose, has been transformed into a sting. 



A pair of many-segmented, antenna-like structures, sometimes short, 

 sometimes long may occur at the end of the abdomen, and these are 

 called cerci. They probably serve as organs of touch, and possibly also 

 of smell in some cases. 



