DIPTERA. 3r)9 



They are either long aucl evenly jointed, as in the Tijyulida', 

 often with long cilia, and sometimes verticilhite, 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 Lopidoptera onl}- a portion of the 

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

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

 maxillae and mandibles, are (when all are "vvell developed, as 

 in the Mosquito) ensheathed partially 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, w'hile the 

 mandibles and maxilh^ are represented b}^ simple setae, though 

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

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

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

 is well, though dift'erently developed, terminating in a large 

 fleshy knob which is divided into two fleshy flaps called the 

 lahelloi. 



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

 hairs may be detected by the microscope on the veins, becom- 

 ing most apparent in the PsAchodie, where the wings are A-ery 

 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 A^ery long and nearly parallel Avith 

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

 gles to it. The A'eins are six in number, and in their direction 

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

 A^enation of the Lepidoptera than any other suborder. The 

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

 doptera, but Avith more cross venules, Avhich in the wing of 

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

 When, as in the Cecidomyiae, the veins become in part ob- 

 solete, only three A'eins 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. 



