12 AN ECONOMIC ENTOMOLOGY. 



Nine rings are assigned to the abdomen, though some of them 

 may be modified into structures faciHtating egg-laying or other 

 special purposes. In the adult no appendages of any kind, ex- 

 cept ovipositors, anal filaments, or cerci, are found ; but in the 

 larva, where there is often no real separation between thorax and 

 abdomen, the latter may bear organs of locomotion which differ 

 in their general appearance and structure from the true legs, and 

 are therefore called "false feet," or pro-legs. The number of 

 these pro-legs sometimes enables us to recognize the order to 

 which a larva belongs, where it otherwise closely resembles 

 another. For instance, all the true caterpillars are larvae of 

 Lepidoptera, or butterflies and moths, and these never have 

 more than four pairs of pro-legs at the middle of the body and 

 one pair on the last segment, making, with the true legs, eight 

 pairs in all. In the larvae of the sawfiies, a family of the Hymen- 

 optera in which the resemblance to caterpillars is very close, there 

 are at least five pairs of pro-legs at the middle and one pair at 

 the end of the body, or no less than nine pairs of legs instead 

 of eight as before. 



No insect has more than two pairs of wings, and these are 

 attached to the second and third thoracic segment. Some have 

 only a single pair, as in the true flies, and then they are on the 

 second or intermediate segment. The prothorax. or first seg- 

 ment, never bears wings. Each thoracic segment bears a pair of 

 legs, which are themselves divided into joints or segments, and 

 their structure will be described more in detail later on. 



This division into three distinct regions and the limitation to 

 six legs in the adult separate the insects from crustaceans, like 

 lobsters, crabs, shrimps, and the like, and from the spiders ; none 

 of which have the head separate from the thorax, while all have 

 eight legs instead of six. An insect in the adult stage is there- 

 fore a jointed animal, the rings, thirteen in number, separated 

 into a head of one, a thorax of three, and an abdomen of nine 

 segments ; moving by means of three pairs of jointed legs. Some 

 differences between "larva" and "adult" have been spoken of, 

 and it has been indicated thereby that at different periods of 

 their lives insects are unlike in appearance. This branch of the 

 subject is exceedingly interesting, and will be treated more fully 

 in Chapter VIII. It is only necessary to say here that, in speak- 



