The First Abdominal Segment of Embryo Insects. 121 
feet structures than their modern equivalents, such as, for instance, the anal 
glands of the Carabidae and the analogous metathoracic organs of the 
Hemiptera. 
Of the three types of pleuropodia, which I have distinguished, the evag- 
inate bulbiform, calyculate, and invaginate, each had its advantages and 
disadvantages, as a secretory organ. The evaginate bulbiform pleuro- 
podium presents extensive~secreting surface but from its prominence and 
necessarily delicate covering it would be readily injured. Situated on 
the abdomen in the median ventral line, or in line with the metathoracic 
legs such organs would, if prominent, be rubbed against the ground or inter¬ 
fere with the movements of the hind legs. This is probably why the pleu¬ 
ropodia move towards the pleurae and project from points outside of and 
near the insertion of the metathoracic legs. This position is also most ad¬ 
vantageous for repugnatory organs. 
The calyculate type, being a transition from the bulbiform to the invag¬ 
inate types, has the tips of the secreting cells protected. The secretion 
may accumulate in the cavity of the organ and be expelled to more advan¬ 
tage when the animal is irritated. The projection of the organ beyond the 
general surface of the body, however, renders it subject to the same injur¬ 
ies as organs of the bulbiform evaginate type. 
The advantages and disadvantages of the pleuropodia of the invaginate 
type are obvious. The glandular cells are efficiently projected, but in 
Zaitha and Cicada the secreting surface of the cells is much reduced. 
Were these glands hollow we should have much more efficient organs, re¬ 
sembling the stinkglands of Julus. It is probable that such hollow invag- 
inated pleuropodia will yet be discovered in some insect embryos. (Hemip¬ 
tera?) 
In the best examples of modern odoriferous glands, like the anal glands 
of the Carabidae, where all the delicate secreting cells are protected by 
being pushed into the body cavity, their being tubular or racemose greatly 
increases the amount of secreting surface, while the presence of a reservoir 
renders it easy for the insect to dispose of a great amount of its malodor¬ 
ous secretion at a moment’s notice. Forms like the larval Papilio with its 
eversible prothoracic gland have all the advantages possessed by the bulbi¬ 
form pleuropodia with none of the disadvantages, since the delicate organ 
can be drawn back into the integment out of the reach of injury. 
The little inefficiencies exhibited by all the pleuropodia as odoriferous 
glands when compared with the more perfect of their modern analogues, 
probably explain why the latter have usurped their places. The pleuro¬ 
podia would, on this supposition, furnish an excellent example of a set of 
organs that have gone down in the struggle for existence and have been re¬ 
placed by organs of more perfect structure though of the same general 
function. 
It is interesting in this connection to cast a glance at Scudder’s tables il¬ 
lustrating the sequence and relative importance of the different orders of 
