DIPTERA. 573 



the hind wings. The latter are composed of a spherical head at the 

 end of a thin stalk. Leydig described at the base of the halteres a 

 ganglion with nervous rods, which he concluded was an auditory 

 apparatus. The head is freely moveable, and usually spherical in 

 form. It is articulated to a short and narrow neck, and is dis- 

 tinguished by the large facetted eyes, which in the male sex 

 may meet in the median line of the face and frontal region. 

 There are as a rule three ocelli. The antennae are constructed on 

 two different types ; they may either be very short and composed 

 of three joints, frequently bearing a tactile hair at the extremity 

 (arista), or they may be filiform and of considerable length and 

 composed of a great number of joints. But since in the first case 

 the terminal joint is again divided into a number of smaller joints, 

 and the tactile hair may be also jointed, it is impossible to draw a 

 sharp distinction between the two types. The mouth parts form the 

 kind of suctorial tube known as a proboscis (haustellum), in which 

 the jaws (mandibles and maxillae) and an unpaired rod (epipharynx) 

 attached to the upper lip may appear as horny, setiform or knife- 

 shaped piercing organs. When the maxillae only are present as 

 paired rods, the unpaired piercing stylet seems to correspond to the 

 fused mandibles. The proboscis, which is principally formed by the 

 labium, ends with a swollen spongy tongue, and is without labial 

 palps, while the maxillae are provided with palps, which, in cases of 

 fusion with the labium, are situated on the proboscis. The abdomen 

 is frequently stalked, and consists of five to nine segments. The legs 

 have five- jointed tarsuses, which end with claws and usually with 

 sole-like lobes for attachment. 



The nervous system presents very different degrees of concentra- 

 tion according to the length of the body. While in flies of very 

 stout build, the ganglia of the abdomen and thorax fuse together to 

 form a common thoracic ganglion; in the Diptera with longer 

 bodies, not only are the three thoracic ganglia distinct, but several, 

 even five or six, separate abdominal ganglia are present. With 

 regard to the alimentary canal, the presence of a stalked suctorial 

 stomach as an appendage of the oesophagus and the number four 

 of the Malpighian tubes may be mentioned. The two tracheal 

 trunks are dilated to two great vesicular sacs at the base of the 

 abdomen. This is correlated with the power of active flight possessed 

 by these insects. 



The male genital organs consist of two oval testes with short vasa 

 deferentia, to which are added firm copulatory appendages. The 



