The First Abdominal Segment of Embryo Insects . 
Ill 
ORDER DIPTERA. 
No pleuropodia have been observed in the following:— 
Family Chironomid^e. 
Chironomus. [Weismann, ("63).] 
Family Tabanid.e. 
Tabanus at rat as, Fabr.? (Wheeler.) 
Family Muscid^ 
Musca [Weismann (’63), Voeltzkow (’89)]. 
ORDER HYMENOPTERA. 
Family Apid.e. 
Apis mellifica, L. 
No pleuropodia [Buetschli (’70), Grassi (’84).] 
The facts accruing from a study of the pleuropodia in the above enumer¬ 
ated forms, representing some of the families of most of the natural orders 
of insects, may be briefly summarized as follows: 
1. The pleuropodia were at one time organs of considerable functional 
importance to the primitive Hexapoda. This is proved both by the size which 
they attain in several cases [ Melolontha , Blatta, etc.] and by the variety of 
structure which they exhibit in different species, sometimes even of the 
same natural order [ Hydrophilus, Melolontha, Meloe.] The latter fact 
would indicate that the organs had occurred very generally among ancient 
insects and had undergone the modification to which the struggle for ex¬ 
istence subjects organs of very general occurrence and important function. 
2. Pleuropodia seem to be of constant occurrence in insects of some 
orders [Orthoptera, Trichoptera (?)], in other orders these organs are as 
constantly wanting [Lepidoptera, Hymenoptera], while in still other groups 
[Coleoptera, Hemiptera] they are well developed in some forms and entirely 
lacking in others. 
3. The pleuropodia are always derived from the ectoderm. 
4. They arise as appendages serially homologous with the appendages of 
the thorax and abdomen. 
5. The pleuropodia described up to date belong to one of two types — 
they are either formed by evagination or invagination. Those of the latter 
type are subspherical and solid; those of the evaginate type appear under 
two forms: the bulbous and the calyculate, the latter being distinguished 
from the former by having the apical area invaginated. [ Acilius, Meloe .] 
The evaginate bulbous form undergoes some modification in different 
species. Thus we may distinguish as its varieties the mushroom-shaped 
[ Gryllotalpa ], thereniform [Oecanthus], the broadly pyriform [Blatta], and 
the elongate pyriform pleuropodium [ Mantis Carolina]. All these varieties 
