122 Wisconsin Academy of Sciences , Arts and Letters. 
insects during geological time, [’86, p. 110 to 113]. Scudder's group of 
Palaeodictyoptera extends from the Silurian to the Trias, culminating in 
the Carboniferous and Permian. This group comprises the generalized pre¬ 
cursors of the more modern Ortlioptera, Hemiptera, Neuroptera, and Coleop- 
tera. It seems not unlikely that some or all of these ancient forms may have 
possessed pleuropodia throughout life, and that specimens may yet be 
found perfect enough to show these organs in the adult. The Orthoptera, 
Neuroptera and Coleoptera proper appear in the Trias,* while the .Hemip¬ 
tera are comparatively well represented in the Lias. Now these more ancient 
orders, constituting the division Heterometabola (of Packard), are just the 
ones, as will be seen from a glance at my table, wdiose embryos pos¬ 
sess pleuropodia. The Diptera, and Hvmenoptera, occurring in the Lias, 
and the Lepidoptera, appearing in the Oolite, have to all appearances kept 
increasing in number and variety up to the present time. In the embryos 
of insects of these orders, comprising the Metabola of Packard, no pleuro¬ 
podia have been observed up to date. 
D. Homologues of the Pleuropodia in the Lower Tracheata. 
A consideration of the pleuropodia of insects embryos would be incom¬ 
plete without a search for their homologues among the lower Tracheata, 
where there are numerous forms with abdominal appendages more or less 
clearly developed. Organs of more or less interest in connection with the 
pleuropodia of insects occur in scorpions, Solifugae, spider embryos, Sym- 
phyla and Thysanura. These cases I will consider in order. 
Cholodkovsky [’89] has called attention to the “combs” of scorpions in 
connection with the pleuropodia of Blatta. These organs, according to all 
accounts, develop as a pair of appendages on the second abdominal seg¬ 
ment and function throughout postembryonic life as sense organs. They 
cannot be regarded as the homologues of the pleuropodia, since there is 
no ground for maintaining a homology between the second abdominal 
segment of the Arthrogastera and the first abdominal segment of the 
Hexapoda. 
Another case of a somewhat similar nature has been made known by 
Croneberg [’87] in Galeodes araneoides . In the just hatched embryo of 
this Solpugid, there is a pair of flat wing shaped appendages about 0.5 mm 
long on the cephalotliorax intercalated between the first and second pairs 
of legs. Their insertions are more pleural than those of the ambulatory 
appendages. Like the pleuropodia of insects, they are pedunculated sacks 
consisting of a single layer of ectoderm cells and contain neither tracheae, 
muscles nor nerves No traces of these peculiar organs are to be found in 
the adult Galeodes. It seems to me very doubtful whether these organs 
* Since the publication of Scudder’s work alluded to, undoubted remains of Coleop¬ 
tera have been found in the Coal Measures of Silisia. 
