54 DIPTERA OF NORTH AMERICA. 



species in which the anterior part of the peristoma is more drawn 

 upwards. 



There is no striking peculiarity in the structure of the thorax. 

 The transverse suture, in agreement with nearly all the Acalyptera, 

 is distinct in the neighborhood of the lateral border, and totally 

 obsolete in the middle of the thorax. The bristles of the thorax 

 not only offer good specific characters, but sometimes also prove 

 very fit for the separation of genera. In that respect the bristles 

 of the middle of the upper side of the thorax deserve attention ; in 

 those species in which their number is the most complete there are 

 three pairs, one before the suture, the second behind the suture, 

 the third a little before the scutellum. More frequently only the 

 second and third pair of these bristles are present, sometimes only 

 the third ; in almost all African species of Dacus they are all 

 wanting. Besides these bristles of the middle of the thorax there 

 are two rows of bristles on each side; the exterior row consists of 

 four bristles, the first of which stands on the humeral callosity, and 

 is often wanting in the Bacina; the second has its place before 

 the transverse suture ; the third, which is often much weaker than 

 the others, in the lateral dilatation of the transverse suture ; the 

 fourth above and a little behind the base of the wing. The inte- 

 rior row consists of three bristles only, corresponding to the three 

 last bristles of the exterior row, but is placed a little more back- 

 wards than these. The scutellum, which is more or less convex, 

 generally bears four stout bristles, but in many Dacina and some 

 Trypetina only two; there is sometimes on each side a weaker 

 bristle between the stout ones. 



The abdomen of the male shows only four distinct segments, the 

 last of which is more or less elongated. The abdomen of the female 

 has five segments before the borer, and the last of them in the 

 Trypetina is always distinct, whereas in the Dacina it is very 

 small, and so concealed under the fourth segment that the abdomen 

 of the female seems to consist only of four segments. That seg- 

 ment which is usually numbered the first, and will also be numbered 

 so in the following descriptions, seems to be composed of two 

 segments soldered together. In many Dacina we also see the next 

 segments more or less completely coalescent on their upper side. 

 The borer of the female is always of a corneous substance ; it is 

 formed of three segments, which are retractile like the drawers of 

 a telescope, and often very long; the last ends in a simple more or 



