14 THE CLASS OF INSECTS. 



insects. In the higher series of orders, comprising the dip- 

 tera, Lepidoptera and Il3'menoptera, placing the highest last, 

 the thorax shows a tendency to assume a globular shape ; the 

 upper side, or tergum, is much arched, the pleural region bulges 

 out full and round, Avhile the legs conceal at their insertion 

 the sternum which is mijiute in size. 



In the lower series, embracing the Coleoptera, Hemiptera, 

 Orthoi^tera, and Neuroptera, the entire body tends to be more 

 flattened ; in the thorax the tergum is broad, especially that of 

 the prothorax, while the pleurites (episterna and epimera) are 

 short and bulge out less than in the higher series, and the ster- 

 num is almost invariably well developed, often presenting a 

 large thick breast-plate bearing a stout spine or thick tubercle, 

 as in (Edipoda. We can use these characters, in classifying 

 insects into orders, as they are common to the whole subclass. 

 Hence the use of characters drawn from the wings and mouth- 

 parts (which are sometimes wanting), leads to artificial dis- 

 tinctions, as they are peripheral organs, though often convenient 

 in our first attempts at classifying and limiting natural groups. 



The abdomen. In the hind body, or third region of the 

 trunk, the three divisions of the typical ring (artlu'omere), are 

 entire, the tergum is broad and often not much greater in ex- 

 tent than the sternum ; and the pleurites also form either a 

 single piece, or, divided into an epimerum and episternum,. 

 form a distinct lateral region, on which the stigmata are sit- 

 uated. The segments of the abdomen have received from 

 Lacaze-Duthiers a still more special name, that of urite, and 

 the different tergal pieces belonging to the several rings, 

 but especially those that have been modified to form the genital 

 armor have been designated by him as tergites. We have 

 applied this last term to the tergal pieces generally. The typi- 

 cal number of abdominal segments is eleven. In the lowest 

 insects, the Neuroptera, there are usually eleven ; as we have 

 counted them in the abdomen of the emlnyo of Diplax. In 

 others, such as the Hymenoptera and Lepidoptera, there may 

 never be more than ten, so far as present observation teaches 

 us. 



The formation of the sting, and of the male intromittent 

 organ, may be observed in the full-grown larva and in the in- 



