BULLETIN 39, UNITED STATES NATIONAL MUSEUM. [6] 



We see, tlieu, that insects share, iu common with many other animals, 

 the jointed or articulated structure. Wherein, then, do they differ? 

 Briefly, in having the body divided into thirteen joints and a snbjoint, 

 including- the head as a joint, and in the adult having six true, jointed 

 legs, and usually, though not always, wings. The live classes of 

 Articulates differ from each other in the number of legs they possess 

 in the adult form, as follows: Hexapoda, 6 legs; Arachuida, 8 legs; 

 Crustacea, 10-14 legs; Myriapoda, more than 14 legs; Vermes, none. 

 This system holds for the adult form only, because some mites (Arach- 

 nida) when young have only G legs, and many true insects in the larva 

 state either have no legs at all, or have additional abdominal legs 

 which are not jointed, but membranous, and are lost in the perfect or 

 adult state. These are called false or prolegs. 



It will serve to make these instructions clear if I at once explain 

 that the life of an insect is marked by four distinct states, viz, the 

 egg, the larva, the jjupa, and the imago, and that the last three words 

 will constantly recur. We have no English equivalent for the words 

 larva and pupa, for while some authors have written them with the 

 terminal e, so as to get the English plural, yet ''larves" and " pupes" 

 so shock the ear that the terms have not been (and deserve not to be) 

 generally adopted. 



We have seen that an insect in the final state has six true legs. 

 Yet even here many species depart from the rule, as there are many in 

 which the perfect insect, especially in the female sex, is apodous or 

 without legs, just as there are also other cases where they are without 

 wings. Sometimes the legs seem to be reduced in number by the partial 

 or total atrophy of one or the other pair, but in all these exceptional 

 cases there is no difficulty in realizing that we have to deal with a true 

 insect, because of the other characters pertaining to the class, some 

 of which it will be well to allude to. 



Insects are further characterized by having usually three distinct 

 divisions of the body, viz. : head, thorax, and abdomen, and by under- 

 going certain metamorphoses or transformations. I^ow, while a num- 

 ber of other animals outside of the insect world go through similar 

 transformations, those in the Crustacea being equally remarkable, yet, 

 from the ease with which they are observed and the completeness of the 

 transformations in most insects, the metamorphoses of this class have, 

 from time immemorial, excited the greatest curiosity. 



SCOPE AND IMPOETANCE OF ENTOMOLOGY. 



But few words are necessary to indicate the importance of entomology, 

 especially to the farming community; for while insects play a most 

 important part in the economy of nature and furnish us some valuable 

 products and otherwise do us a great deal of indirect good, yet they are 

 chiefly known by the annoyances they cause and by the great injury 

 they do to our crops and domestic animals. Hence some knowledge of 



