Introduction to the S^tiopsis of the Acrididce. 255 



yet those who are but beginners in entomology are frequently puzzled, 

 in reading the description of a part of the body of an insect, to know 

 in what sense the author uses these terms ; for example, whether 

 "length" means in the direction of the longest diameter of the part, 

 or in the direction of the length of the insect ; but by knowing that 

 these terms are always used with reference to the whole insect, there 

 will be no difficulty in understanding the description. It often hap- 

 pens that by following this rule rigidly the width of a part exceeds its 

 length, yet this must not cause us to vary from it. 



The body of the insect is naturally divided into three distinct parts, 

 each bearing certain appendages : 



First- — The head and its appendages, the antennce and palpi. 



Second — The thorax and its appendages, the wings and legs. 



Third — The abdomen and its appendages, the cerci and ovipositor. 



The head varies considerably in shape, yet the t}'pical form may be 

 considered an oblate spheroid, with the longitudinal diameter (from 

 the neck and through to the face) the shortest, and its perpendicular 

 diameter the longest. In some species the upper portion of the 

 front is prolonged into a cone or pyramid of greater or less length (as 

 in Tryxalis Opotnala^ etc.) ; in one genus (Acrolophitus) the vertex rises 

 obliquely upward, in the form of a short cone or pyramid. Viewed 

 from the side, it presents, in many species, a triangle — the face, which 

 form<? the longest line, being directed ft'om the vertex obliquely under 

 toward the breast {Opomala, O.xycoryp]ms, Stenobothrus, etc.); in other 

 species it presents a parallelogram, the greatest length being up and 

 down — (Edipoda, Acridiiim, Calopteims, Pezotettix, etc. 



In describing the head, its external surface may be considered with 

 reference to four planes — the dorsum, the two sides, and the front. 

 If a line be drawn across the back part, from the posterior margin of 

 one eye to the posterior margin of the other, the portion lying behind 

 this line, reaching to the marghi of the pronotum, will represent the 

 occiput. 



The vertex is the portion included between the eyes, and extends 

 forward to the point where the. head commences to descend to the 

 face ; the extreme anterior point is sometimes called the fadigium. 

 The variations in the form and surface of the vertex afford important 

 generic and specific characters. In a few instances it ascends anterior- 

 ly, is somewhat horizontal, but is generally more or less deflexed ; is 

 advanced and pointed or triangular in front {Tryxalis, Opomala, 

 Stenobothrus, Tragocephala) ; is obtusely rounded and blunt without 

 carvings {Boopedon) ; is narrow and slightly furrowed {Caloptemis^ ; 

 and is broad and even transverse, that is, broader than long. Some- 



