4 INTRODUCTION [CH. 



pupal or resting-stage, but with a considerable change of structure 

 at metamorphosis. 



Imago. Head with biting mouth-parts, the mandibles thick 

 and strongly toothed; antennae reduced, filiform; three ocelli, 

 two large compound eyes. Thorax with small movable prothorax, 

 and large obliquely-placed synthorax (formed by fusion of meso- 

 and metathorax) ; the legs placed well forward and of little use 

 for walking ; the wings placed far backward ; two pairs of thoracic 

 spiracles. Wings, two equal or sub-equal pairs, unfolded, richly 

 veined ; the six typical longitudinal veins all present, but R + M 

 and also Cu + A fused basally for a short distance ; a secondary 

 anal vein (A') developed; Sc stopping far short of the apex, at 

 a thickened cross- vein, forming a joint, the nodus; a thickened 

 membranous patch or pterostigma nearly always present between 

 C and R, near the apex; R either simple or once-branched; M 

 branched either three or four times ; an arculus and discoidal cell 

 always present. Abdomen with ten complete segments, and ter- 

 minal appendages; in shape, narrow, elongated, the pleurites 

 membranous, the large tergites enfolding the reduced sternites; 

 eight pairs of spiracles. External Genitalia : <J with genital pore 

 on ninth sternite; complicated copulatory apparatus developed 

 from the second and anterior part of the third sternites. ? with 

 genital pore in the suture between eighth and ninth sternites; 

 ovipositor either complete or reduced. Mid-gut without caeca. 

 Numerous Malpighian tubules. 



Larva. Similar to imago in general features, but with less 

 elongated and often very broadly rounded abdomen. Compound 

 eyes present from birth; ocelli absent during most of larval life. 

 Antennae larger than in imago. Labium very highly specialized, 

 forming an elongated jointed mask for the capture of prey. 

 Legs strong, longer than in imago. Wing-sheaths developed 

 externally and gradually, the hind- wing coming to overlie the 

 fore- wing; developing wings richly supplied with tracheae, 

 around which the main venational scheme is subsequently formed. 

 Tracheal System secondarily closed; the spiracles present, but 

 either closed or only partly functional. Gills developed in the 

 rectum, or on the anal appendages ; rarely also as lateral appen- 

 dages on some of the abdominal segments. 



