177 



anterior extremity of this plate, another, smaller plate, which is an- 

 teriorly curved downward and consists of two — rarely one — hooks or 

 mandibles which are either simple or toothed and operate vertically, 

 scraping or abrading the surface of the larval pabulum so that the 

 food is drawn downward and inward to the mouth cavity. These 

 mouth-hooks, and the entire cephalopharyngeal skeleton, are retracted 

 when the larva is at rest. The respiratory system consists of pro- 

 thoracic and anal spiracles which are connected by means of two large 

 main tracheae, the latter being normally connected by means of a slen- 

 der transverse trachea just behind the prothoracic spiracle, and each 

 sending out upon each segment a stout downwardly and slightly for- 

 wardly directed stout trachea with numerous branches, and, in addi- 

 tion, a smaller trachea, which is an offshoot from the inner surface 

 and is directed almost straight cephalad. The structure of the spira- 

 cles is of great value as a character in the classification of this group. 

 The anterior spiracles are often cjuestionably functional in the aquatic 

 forms, and are sometimes almost or entirely absent in parasitic species, 

 the connection with the air being maintained entirely by means of the 

 posterior pair. Means of locomotion in this group consist of spines 

 on various portions of the thoracic and abdominal segments, with, 

 occasionally, poorly developed pseudopods on the abdominal segments, 

 particularly on the apical segment. Rarely there are elaborate appen- 

 dages upon the thoracic and abdominal segments (Fanniinae), but 

 more frequently the surface of the segments is bare except for the 

 locomotor organs. The larvae transform to the pupal stage within 

 the indurated last larval skin, which is then referred to as the puparium. 

 In this stage characters are frequently developed that are of value in 

 classification ; these are dealt with in the section upon characters of 

 the pupae. 



Characters of the Pupae 



Suborder OETHOEEAPHA 

 Division Xem.\tocera 



The aquatic members of this group may in the main be separated 

 from those of the Brachycera by their stalked prothoracic respiratory 

 organs, which are occasionally numerously filamented apically. The 

 few species that are recorded as having these organs sessile may be 

 distinguished from brachycerous pupae by the peculiar recurving of 

 the legs against the ventral surface of the. base of the abdomen and 

 the posterior portion of the thorax (Chironomidae). The terrestrial 

 species are distinguishable from brachycerous forms by the very long 

 antennal sheaths which curve over the upper margin of the eyes ; 



