1935 ] 
Structure of the Pretarsus 
7 
distitarsus over which the unguitractor slides, is well de- 
veloped in Odonata and a process of this structure extends 
beyond the bases of the ungues (un) . The only other orders 
in which this condition has been observed are Coleoptera 
and Hemiptera. 
Order Ephemeroptera 
Siphlonurus sp. (Fig. 2) 
The unguitractor is a small pear-shaped plate and the 
claws (un) are simple. The important feature in this case 
is the membranous pad-like structure (pv?) closely applied 
to the ventral surface of each of the ungues. It is this con- 
dition which has led Crampton to suggest that possibly the 
pul villi (Diptera) were derived from some such situation, 
that is, a splitting off of a ventral membrane from the claw 
itself. Proximally these pad-like structures are closely as- 
sociated with the membranous areas transmitting tension 
from the unguitractor to the claws and’in fact give evidence 
of being continuous with them. 
Although these examples of Odonata and Ephemeroptera 
both show their elementary position, (absence of arolium, 
planta, basipul villi or other significant structures), they 
give no evidence of any close relationship for the presence 
of an empodium in Anax indicates a certain amount of spe- 
cialization. Thus, if these cases prove to be typical, the con- 
clusions to be derived from the comparative morphology 
of the pretarsus will evidently be in agreement with the 
general conception of the relative positions of these orders. 
Cursorial Orthoptera 
Blattidae — Periplanta americana L. (Fig. 3) 
The arolium (ar) is large in this family, although its 
structure is simple compared with the complicated types 
found in the higher orders. Ventrally it is membraneous and 
concave, so that it forms a most efficient adhering organ. 
The unguitractor is typically large and heavily sclerotized. 
Two other structures of importance appear in this group. 
The planta (pi) is a median quadrate plate just distal to 
the unguitractor and seems to be lightly sclerotized; it 
bears a single large seta. On either side of the planta is a 
