GENERAL STRUCTURE 9 



and fully illustrated work is very valuable for reference, and contains an 

 extensive bibliography. Most of the larger works on zoology contain a 

 certain amount of general information on the subject, dealt with mainly 

 from the comparative point of view. To those who have no previous 

 acquaintance with structural entomology Miall and Denny's account of 

 the cockroach is strongly recommended ; the authors have used their 

 type to set forth in a masterly fashion the main points in the anatomy 

 and physiology of insects, and there could be no better preparation for 

 work with insects than the study of this admirable little book. 



A list of papers dealing with particular forms is given at the end of 

 this chapter, and the worker may consult these as occasion arises. For 

 further references, especially to the older and less accessible literature, 

 the bibliography in Berlese's book, or the lists given in the special papers, 

 may be consulted. Most of the papers of practical value are compara- 

 tively recent, and are to be found in the larger libraries. 



Insects present the characteristic features of the phylum ARTHROPODA, 

 that is to say, they have a more or less elongated body, with a mouth 

 at one end and an anus at the other, a central nervous system concen- 

 trated in the head, and a ventral chain of ganglia; the body is metameri- 

 cally segmented, and each segment bears typically a pair of jointed 

 appendages ; the body cavity is not a true ccelom, but a haematocoele, in 

 communication with the dorsal blood vessel. The class INSECTA is 

 specially distinguished by the separation of the body into three well- 

 defined regions, the head, thorax, and abdomen ; only three pairs of 

 legs are present, and there are usually two pairs of wings. The order 

 DIPTERA is separated from other insects chiefly by the transformation 

 of the hind pair of wings into the structures known as halteres or 

 balancers, and by the adaptation of the mouth parts to form a sucking 

 organ. 



By metameric segmentation is meant, that the body of the animal is 

 made up of a number of separate segments or metameres, arranged one 

 behind the other in the long axis of the body. (Plate I, fig. 3.) In insects 

 there are said to be typically about twenty segments, of which the first 

 six go to form the head, the next three to the thorax, and the remainder 

 to the abdomen. Each segment should bear a pair of appendages, but 

 in the adult insect only those on the head and thorax remain, and those 

 on the head are always very highly modified. It is possible that certain 

 2 



