XX INTRODUCTION. 



ABDOMEN. 



The abdomen is composed of a variable number of segments, 

 more or less closely fused together. The normal number for 

 insects, nine, are rarely all visible (the Tipulidse are exam- 

 ples). They are counted from the base on the upper side. In 

 not a few cases the first two are so closely fused together, and 

 the first one abbreviated, that the nomenclature leaves some 

 doubt in the mind of the student. The upper part of the ab- 

 domen may be especially indicated by the word dorsum, but 

 in general, the venter or under part is alone thus contradistin- 

 guished. The male genitalia, which in many cases are of 

 complicated structure, and of much value in classification, are 

 known collectively as the hopopygium, though Bergroth has 

 proposed the name propygium for them. The ovipositor of the 

 female abdomen very frequently projects from the abdomen, 

 and is sometimes extremely long ; its structure is often char- 

 acteristic of genera or families. The more detailed descrip- 

 tion of the parts of both these organs it is unnecessary to go 

 into here ; they may be studied in the different families, where 

 they often find useful application in the separation of species. 



CH^ETOTAXY. 



Until within recent years but little attention had been paid 

 by writers on systematic dipterology to the number and ar- 

 rangement of the bristles among flies. Osten Sacken, who 

 proposed the term chcvtotaxy, to designate the science of their 

 arrangement, published an epochmaking paper on the subject 

 in 1881. Since that time their use has steadily increased 

 with the most happy results. That they will become more 

 and more useful as their study in different families progresses, 

 there can seem to be no doubt. For that reason, it behooves 

 the student to become familiar with the nomenclature already 

 in use. The following are the most important terms : 



CEPHALIC BHISTLES. 



Vertical, two pairs, inner and outer, inserted more or less 



