DIPTERA. 359 



They are either long and evenly jointed, as in the Tij^iclidcey 

 often with long cilia, and sometimes verticillate, as in Ce- 

 cidomyia ; or, as in the House fly, the typical form is a 

 short and stout, two to three-jointed antenna, ending in a 

 bristle. 



In the Hymenoptera and Lepidoptera only a portion of the 

 mouth parts are used for sucking in food, but in the present 

 group, the labruni, with the two pairs of appendages, i.e., the 

 maxillte and mandibles, are (when all are well developed, as 

 in the Mosquito) ensheathed partiall^^ within the labium, and 

 with the last form a channel for the passage of the fluid food 

 into the mouth. 



The labium forms the under side of the sheath, while the 

 mandibles and maxillne are represented by simple setfe, though 

 the one, two, or three-jointed maxillary palpi are present, and 

 in this last character the rostrum of the flies differs from the 

 beak of the Hemiptera. As in the Hymenoptera, the lingua 

 is well, thougii diff'erently developed, terminating in a large 

 fleshy knob which is divided into two fleshy flaps called the 

 labellm. 



The wings are naked, as in the Hymenoptera, though fine 

 hairs may be detected by the microscope on the A'eins, becom- 

 ing most apparent in the Psychodae, where the wings are very 

 hairy. In form they are long and narrow, the costal edge 

 being straight, the apex of the wing obtusely rounded, while 

 the oblique outer edge is very long and nearly parallel with 

 the costa, where in the Lepidoptera it is nearly at right an- 

 gles to it. The veins are six in number, and in tlieir direction 

 and branches (Fig. 270-271) correspond more closely with the 

 venation of the Lepidoptera than any other suborder. The 

 veins are straight, and with fewer branches than in the Lepi- 

 doptera, but with more cross venules, which in the wing of 

 the Tijyulidm, remind us of the net-veined Neuroptera. 

 When, as in the Cecidom^dte, the veins become in part ob- 

 solete, only three veins remain, the costal, subcostal and 

 median. The form and size of the cells, especially the submar- 

 ginal ones, are of much use in distinguishing the species, while 

 the changes in the costal and basal portion of the wing are 

 the most important in classifying the genera and families. 



