NORTH AMERICAN DIPTERA. 



321 



median or labial stiuctures. At b, the galea and the lacinia, palpifer 

 and palpus of one side are shown as they appear separated out, and 

 at c, the remainder of the maxillary structure appears. It is inter- 

 esting to note that the palpi are furnished with a sensory pit as in 

 SimuUum, of which, indeed, this is likely a member. Easily sepa- 

 rable from the maxilla, without the destruction of any chitinous parts 

 is the labium, shown at d. In this the ligula, while obvious in the 

 preparation, is not separated out, because I could not manipulate 

 my needles under the high power necessary. The paired natui-e of 

 the paraglossse is obvious, and this could not have anything to do 

 with the labrum, from the simple fact that I do not know of a case 

 in which the labrum is paired, and of no case in which it is an in- 

 ternal mouth organ. The labrum must also be articulated t<) the 

 clypeus at base, if homologies are to be preserved, and this clypeus 

 must then be the square plate seen in figure 4, c, and again in the 

 figure of Tabanus atratus. This would bring the clypeus entirely 

 inside of the head, where it does not belong, Avhile if we take it to 

 be the mentum, not only is its place natural, but the attachments to 

 it have a natural significance, and the development is an easily ex- 

 plicable one ; which is certainly not so if we refer to the labrum 

 shown in figure 1, and are compelled to imagine this modified into a 

 tubular or channelled prolongation. This interpretation also makes 

 it unnecessary to account for the development of a fleshy, sensory 

 organ, into a chitinous, mechanical structure. 



Leaving the development of the galea for a moment, an examina- 

 tion of the mouth parts of 

 Tabanns will be interesting. 

 Fig. 6 represents the max- 

 illae and labium of the large 

 black species which I take 

 to be atratus. In examining 

 the central organ here, we 

 find the broad basal ])la.te 

 representing the mentum, 

 the united paraglossse,which 

 here show no observable 

 suture as in Simulium, the 



Fig. 6. Moulh parts o( Tadanusairaius. central ligula, and, UlOSt 



interesting of all, the rudimentary labial palpi, closely applied to the 

 sides of the. paraglossse, but i-eaching st)me distance beyond them 



