132 ZOOLOGY. 



The order includes the families, Scutigeridce, Lithobiidm^ ScolopendridcB^ 

 and Geophilidca. 



The genus Scolopendra has four pairs of eyes, twenty-one segments, or 

 twenty-two if the head is considered to be composed of two segments. In 

 the latter case the segments may be made to correspond with the thirteen 

 composing the body of insects, if the prsescutum, scutum, scutellum, and 

 postscutellum of each of the three thoracic segments, are counted separately. 

 Under tliis view, the segment preceding the nine abdominal segments in 

 Scolopendra will be the metathoracic postscutellum ; and the posterior 

 division of the head will be the prothoracic praescutum. A similar division 

 of tlie segments appears in Cryptops. 



The genus Scolopendra is widely distributed over the globe, the larger 

 species (one of which is a foot in length) being peculiar to warm regions. 

 Their bite is poisonous, and may be compared to that of the scorpions. 



Class 6. Insecta. 



The name of the class of Insects is derived from the insected or articu- 

 lated structure of the body, and its frequent division into several portions, 

 as in the Hymenoptera. It has been variously applied to portions of the 

 Articulata, but always including the hexapod orders, which are provided 

 with wings in most cases, and to which the term has been more and more 

 restricted. 



Insects are dioicous articulate animals, breathing air by means of tracheae, 

 and having: a head and abdomen united bv an intermediate thorax beariuo- 

 the six feet and two or four wings when these are present. They have a 

 free head bearing two antenna?, and they are subject, during their growth, 

 to certain external and internal changes termed metamorphoses. Most 

 insects have wings, a peculiarity which none of the other classes possess. 

 The integument is usually suflSciently hard to serve as a kind of external 

 skeleton, to the inside of which various muscles are attached. 



The body of insects is usually considered to be composed of thirteen 

 (sometimes fourteen) segments, which are aj^parent in the larva, although 

 some of them are frequently so much reduced in size, or so intimately joined 

 together, that they cannot be distinguished in the adult. Tlie head forms a 

 single segment, followed by the thorax, which is composed of three seg- 

 ments, and the remaining ones belong to the abdomen. 



In Orismology, or the application of names to organs, it has become a 

 matter of very great importance to apply the same name to the same part 

 in different groups of animals, so far as this can be satisfactorily ascertained. 

 The neglect of the older entomologists to observe a rule the advantages of 

 which ai*e so apparent, has been productive of much confusion, and we 

 accordingly find the term thmmx^ which is correctly applied to the part 

 between the head and abdomen of a Rymenopter {pi. Y9, figs. 11, 14, 18), 

 also employed to signify the segment next to the head in Coleoptera 

 336 



