INSECTS. 



89 



Class II.— INSBCTA. 



The term Insecta is used with vai'ying significations by different authors, some 

 restricting it to the six-footed forms, while others enlarge it so as to include the spiders, 

 mites, and millepeds as well, reserving the term Hexapoda for the Insecta of the 

 former. 



In the present work the term is used in its most comprehensive form, and includes 

 all arthropods in which the body is more or less clearly divided into three regions, — 

 head, thorax, and abdomen ; which respire by trachea instead of gills ; undergo a more 

 or less complete metamorphosis in passing from the embryonic to the adult condition ; 

 and which have, at no stage of development, the two-branched appendages which we 

 have found so common among the Crustacea. 



The variety of forms contained in the Insecta is so great that only the most general 

 statements can be made regarding the class as a whole, and the more special characters 

 are hence reserved for mention under the different divisions. 



The body of insects, like that of all arthropods, is made up of a series of segments 

 ranged one after another, forming a body of varying length. The number of these 

 segments A'^aries greatly, there 

 being, for instance, ten in some 

 Myriapoda, and two hundred in 

 others of the same group. Each 

 of these segments (and the same 

 is also true of Crustacea) is re- 

 garded as made up of eight por- 

 tions, the relations of which may 

 be seen in the accompanying 

 diagram. Of these segmental 

 elements the two upper are 

 the terga (singular tergum), the 

 upper lateral ones are called 

 epimera, the lower lateral epi- 

 sterna, while those on the ventral 

 surface are known as the sterna. These elements are variously developed in the dif- 

 ferent forms. Between the sterna and the episterna arise the legs and their various 

 modifications, which, as has just been said, are composed of but a single series of joints, 

 and never present a two-branched condition in either embryo, larva, or adult. Be- 

 tween the episterna and the epimera are the openings to the respiratory system, a 

 feature which is entirely confined to the group now under discussion. In the six- 

 footed forms there are additional locomotive organs, the wings, never more than two 

 pairs, and these when present are inserted between the epimera and the terga. 



The body of insects is usually divisible into regions or groups of segments. In the 

 typical six-footed forms we have three of these divisions known as head, thorax, and 

 abdomen. In the spiders the head and thorax are more or less completely united into 

 a cephalothorax, while the abdomen remains distinct. In the myriapods the head is 

 well distinguished, but the remaining segments are so similar that no differentiation 



Fig. 124. — Section through the thorax of an Insect, a. Alimentary 

 canal, b. Tergum. e. Episternum. A. Heart. I. Leg. m. Epimerum. 

 ». Nervous cord. s. Spinacle. *. Trachea, v. Sternum, w. Wing. 



