26 Dana on the Classification of Animals 
8. The forms among the Lepismians are related to those of 
Myriapods, as has been observed by different writers, and so also 
are their movements. Thus they occupy a position between 
Insects and an inferior order of Insecteans. 
4, The third or degradational group of Insects, if such there be, 
should contain, according to analogy, elongated larve-like forms, 
such as make an elementalized exhibition of the Insect-type. As 
the longicaudate Birds, or Erpetoids, constitute the third or 
degradational division of Birds (aérial Vertebrates), so the longl- 
caudate Thysanures may well represent the degradational divi- 
sion of Insects (aérial Articulates). The shorter Podurians are 
elliptic forms. 
5. While Insects of the first grand division are prosthenic, and 
those of the second are metasthenic, those of the third are, on the 
scheme proposed, urosthenic, even those few which are not salta- 
torial using the caudal extremity in locomotion. It accords with 
the relations in many other departments of the animal king- 
dom that these three sthenic grades should mark off the three 
grand divisions. 
6. With regard to the exclusion of other apterous Insects, we 
offer the following remarks. The apterous Pediculi, as Nitzsch 
long since observed, have no characteristics that would separate 
them from Hemipters, and the-Nirmids none that would remove 
them from Orthopters. They are simply inferior wingless spe- 
cies of those types, as much as the Coccids are of Homopters; 
and they have nothing of the agility of the Lepismids, ‘There 
are no points of structure indicating an affinity to any two or 
more of the higher subdivisions of Insects, or to the inferior 
Myriapods; they are not urosthenic, being in no way essentially 
different, as regards their legs, from the types to which they are 
referred, — 
Fleas are permaturative, like all Apipens, and in this and other 
ways show that they have no relations to the Lepismians. e 
reasons for regarding them as an independent type under the 
Apipens have been presented on page 18. 
The Lepismians and Podurians appear therefore to be rightly 
made the third grand ‘group of Insects. Like the Erpetoid 
birds, and degradational or intermediate types in other cases, 
the group may have been well-represented in species in t 
geologieal ages. At the present time we know of only the two 
above-mentioned families under this type,sand both are sup- 
i to have eloser relations to the Pteroprosthenics than to the 
terometasthenies, If any group ever existed related as closely 
to the Pterometasthenies, as the above mentioned are to the 
_ Pteroprosthenics, and if, besides, there has existed a third typical 
group, the species are yet to be discovered, either fossil or living- 
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