1935 ] 
Structure of the Pretarsus 
5 
of most of the various orders observed show a plate which 
is probably homologous with the hymenopterous planta and 
therefore this term is used wherever practicable. Coleop- 
tera, Diptera, Hemiptera, Dermaptera, Odonata and Eph- 
emeroptera do not exhibit the planta in its typical condi- 
tion although an empodium is present in these orders (ex- 
cept Ephemeroptera) which suggests the possibility that 
the empodium is merely a modified planta, closely associ- 
ated with the unguitractor. Further study is needed to de- 
termine the correct relationships of these structures. 
Snodgrass (1929) suggests that the planta may possibly 
be a subdivision of the unguitractor. However, the nature 
of the planta under high magnification is unlike that of the 
unguitractor which always gives a granulose appearance, 
is heavily sclerotized, and is sharply demarked in every 
case observed even to the primitive Odonata and Ephemer- 
optera. On the other hand the planta is variable in extent 
and degree of sclerotization and never shows the heavy, 
rugose aspect of the unguitractor. It seems more likely 
that the planta represents merely an area of the mem- 
branes distad of the unguitractor which has become sclero- 
tized in greater or less degree according to the stress to 
which it is subjected by the tension of the apodeme and un- 
guitractor. Of course the unguitractor itself is a sclerotized 
area of the membrane but it is always distinct in outline 
and not an indefinite structure as the planta often appears 
to be. (Note. — Planta has also been used to refer to the 
basal joint of the post tarsus in pollen-gathering Hymen- 
optera, to the soles of the post tarsal joints and to the anal 
clasping legs of caterpillars). 
Pseudarolia . — Paired structure which occurs beneath the 
claws of some Hemiptera (Miridae) and which possibly will 
prove to bear some relation to pulvilli. 
Pulvillus . — Originally referred to the pad-like structure 
beneath each claw in many Diptera. Other orders in which 
undoubted homologous parts occur are Lepidoptera and 
Trichoptera. Membranous areas are found beneath the 
claws in most orders but it is impossible to consider them 
as homologous with the pulvilli. In this paper they are 
