1935 ] 
Structure of the Pretarsus 
17 
ton 1923) . The arolium of Tabanus is simple and lacks the 
supporting sclerite, or camera. For this reason there may 
be some question as to its strict homology with the arolium 
of Mecoptera, etc. described above. The absence of a 
planta may be significant as a similar absence was noted 
in Mantispid Neuroptera. On the other hand, the arolium 
is like; that of Ithone in nature i. e., its ventral surface is 
continuous with the membrane distad of the unguitractor) 
and the planta of Ithone is so lightly sclerotized that it is 
conceivable that in many related forms the planta would be 
entirely absent. 
Asilidae — Diogmites umbrinus Loew. (Fig. 24) 
This insect has no arolium but is characterized by the 
presence of a setiform empodium, a long chitinous setiform 
structure arising from the distal margin of the unguitrac- 
tor. The pulvilli are also long and narrow and similar to 
those of Trichoptera and Lepidoptera, but the basipulvilli 
are small. 
A study of more primitive Diptera may bring out some 
annectant features which will be significant in regard to 
the origins of this order. Thus far there seems to be more 
common features in the Neuroptera than in the Mecoptera. 
Order Hymenoptera 
Siricidae — Tremex columba L. (Fig. 25) 
The unguitractor is long and grooved and the basipul- 
villi are very small; pulvilli probably do not occur in the 
Hymenoptera. Tremex lacks an arolium which is a com- 
plicated structure when present in this order. The planta 
bears four setae and the claws are provided with a large 
median tooth or spur. The orbicula (rb) is a dorsal scler- 
ite which is here shown only in part. 
Sphecidae — Chalybion csereuleum L. (Fig 26) 
The arolium, here shown extended, is normally folded 
back against the planta when not in use. Except for the 
paired lobes this structure is much like that of Panorpa 
especially in reference to the camera. The planta is pro- 
vided with many small setae and the unguitractor differs 
