1935 ] 
Structure of the Pretarsus 
19 
tera and Hemiptera may also point to a close relation be- 
tween these structures. These considerations, if supported 
by further investigation, indicate that the empodium is 
really a specialized planta, i. e., a planta which has become 
closely associated with the unguitractor and in some cases 
lengthened into a setiform process. 
3. That the arolium or median pad-like structure occur- 
ring in many insects, although varying in complexity, is 
homologous throughout the orders. This structure is not 
of ordinal significance for it is present in some groups and 
absent from others within most orders. 
4. That the small lateral sclerites (basipulvilli) are ho- 
mologous in all orders ; they form a support at the bases of 
the pulvilli when the latter are present and in such cases 
there is no question as to their homology. Many insects 
show these sclerites without the pulvilli but all other rela- 
tionships are the same. 
5. The problem of the origin of the pulvilli remains in 
doubt. Very possibly further study will support the sug- 
gestion of Crampton (1923) that they are membranous 
areas detached from beneath the claws. Thus far what 
evidence there is remains inconclusive, that is, briefly: the 
membranous claws of Ephemeroptera and the similar con- 
dition in certain Homoptera (Fig. 15) ; the pseudarolia of 
Miridse which may be true pulvilli; the association of pul- 
villi and basipulvilli in Diptera, Trichoptera and Lepidop- 
tera and the fact that the basipulvilli (if present) are al- 
ways located at the bases of the flexor membranes in orders 
where typical pulvilli do not occur. 
6. An interesting condition is that found in the orthop- 
teroid insects. The similarity of certain anatomical fea- 
tures of the Blattidse, Mantidse and Isoptera is well known 
so that the agreement found in the claw-segments of these 
three groups is not surprising. The saltatorial Orthoptera, 
including the Phasmidse, are also set apart as a coherent 
group by the nature of the pretarsus and the Dermaptera 
would seem to present a third Orthopteran type although 
less distinct and nearer to the Tettigoniids and .Gryllids- 
plus coleopterous affinities. 
