ABDOMEN 



109 



exist in a more or less rudimentary or vestigial condition, though 

 they are never vised for purposes of flight. 



The abdomen, or hind body, is the least modified part of the 

 body, though some of the numerous rings of which it is composed 

 may be extremely altered from the usual simple form. Such 

 change takes place at its two extremities, but usually to a much 

 greater extent at the distal extremity than at the base. This 

 latter part is attached to the thorax, and it is a curious fact 

 that in many Insects the base of the abdomen is so closely 

 connected with the thorax that it has all the appearance of 

 being a portion of this latter division of the body ; indeed 

 it is sometimes difficult to trace the real division between the 

 two parts. In such cases a further differentiation may occur, 

 and the part of the abdomen that on its anterior aspect 

 is intimately attached to the -thorax may on its posterior 

 aspect be very slightly connected with the rest of the abdomen. 

 Under such circumstances it is difficult at first sight to recognise 

 the real state of the case. When a segment is thus transferred 

 from the abdomen to the 

 metathorax, the part is 

 called a median segment. 

 The most remarkable 

 median segment exists in 

 those Hymenoptera which 

 have a stalked abdomen, 

 but a similar though less 

 perfect condition exists in 

 many Insects. When such 

 a union occurs, it is usually 

 most complete on the dorsal 

 surface, and the first ventral plate may almost totally disappear : 

 such an alteration may involve a certain amount of change in 

 the sclerites of the next segment, so that the morphological deter- 

 mination of the parts at the back of the thorax and front of the 

 abdomen is by no means a simple matter. A highly modified 

 hind-body exists in the higher ants, Myrmicidae. In Fig. 60 

 we contrast the simple abdomen of Jaigyx with the highly modi- 

 fied state of the same part in an ant. 



Unlike the head and thorax, the abdomen is so loosely knitted 

 together that it can undergo much expansion and contraction. 



Fio. 



-Simple abdomen of Japyx (A) contrasted 

 with the highly modified one of an ant, Crypio- 

 cerus (B). The segments are numbered from 

 before backwards. 



