406 THE CANADIAN ENTOMOLOGIST 



unlike those of most Nematoceran genera, live entirely, and con- 

 tinually, below the surface of the water, they are apneustic, and all 

 of the oxygen they require has to be extracted from the water 

 through three supra-anal finger-like gills. The relatively small 

 size of these accounts, in all probability, for the necessity ol these 

 larvae living in fast flowing water, for when they are placed in still 

 water they soon die, presumably of asphyxiation. 



In maturing larva? the histoblasts of the pupal and adult 

 organs are well developed and most conspicuous. Thus on each 

 side of the thorax can be seen the three leg-, the wing-, and the 

 halter-histoblasts, as distinctly limited whitish areas. The pupae 

 of these flies resemble the chrysalids of the Heterocera with the 

 exception of having al' the spiracles closed, and the respiratory 

 function being accomplished by a tuft of respiratory fila- 

 ments situated on each side of the prothoracic region. These 

 project far out of the slipper shaped cocoon in which the pupal 

 stage is passed. The histobbsts of these filaments turn black in 

 the later stages of larval development, and, when the latter assumes 

 the chestnut brown colour of maturity they appear as a black tri- 

 angular area on each side of the prothorax. 



The commonest Simuliid around Boston, in Spring, is Simulium 

 hirtipes. In the larva of this species two classes of parasites occur. 

 One of these is represented by a nemathelminth worm, belongihg 

 to or near the genus Mermis. The worm lives either singly, or in 

 considerable numbers, coiled up within the body cavity of its host, 

 where it occupies the ventral portion of the somewhat swollen ab- 

 dominal region (PI. XV, fig. 1). When one worm only is present it 

 measures about three centimetres, which is nearly three times the 

 length of its host. The greatest number of worms found in a single 

 larva was twelve. In this case none attained to a greater length 

 than 1 cm. The most striking efl'ect of these parasites upon their 

 larval host is that they so far inhibit the development of the histo- 

 blasts that pupation becomes impossible (fig. 2). This suppression 

 ' of pupal and adult organs is accompanied by a slight increase in 

 the size of the larval tissues, for most parasitised larva? were from 

 2 to 3 mm. longer than their healthy companions. This condition 

 is opposite to Prothetely, which name Kolbe ('03) ascribed to the 

 several recorded cases in which larvae of various orders had their 



