INTERNAL ANATOMY. 49 



are : erect, when standing perpendicularly or nearly so ; 

 proclinate, when directed forward ; reclinate when directed 

 backward; divaricate or divergent when directed outward 

 from the middle line ; convergent when directed inwardly; 

 decussate or cruciate when crossing each other. 



INTERNAL ANATOMY. 



For the following brief account of the internal anatomy 

 of Diptera I am indebted to Prof. V. L. Kellogg. 



The special features of the internal structure of the 

 Diptera are the high degree of concentration of the nerv- 

 ous system attained in some of the members of the order, 

 the expansion of the two main tracheal trunks in the base 

 of the abdomen to form air sacs, the presence of a suck- 

 ing stomach as in the L,epidoptera, the constant number 

 (four, rarely five) of the Malphigian tubes, and the ab- 

 sence of a bursa copulatrix in the females. 



The alimentary canal presents behind the oesophagus 

 an expansion which is a crop or sucking stomach. The 

 ventriculus, or true stomach, lying behind it, has usually 

 two caeca; and the long, slender, Malphigian vessels are, 

 in almost all species, four in number, a surprising con- 

 stancy compared with the condition in other groups of 

 specialized insects. The vessels open singly into the ali- 

 mentary canal in some flies and in others they unite in 

 pairs before reaching the canal and open into it by two 

 ducts. 



The heart is of the usual type, but with only two cham- 

 bers in the more specialized families, owing to the con- 

 centration of the body. In the larva of Corethra the heart 

 is a simple, elongate tube without chambers. 



The two main tracheal trunks expand at the base of 

 the abdomen into conspicuous air sacs similar to those 

 found among Hymenoptera, L,epidoptera, the lamellicorn 

 beetles and some other insects. The two pairs of spira- 



