24 DEFINITION OF THE TERM INSECT. 
belonging to the last-mentioned class, has nevertheless a 
distinct thorax consisting of more than one piece, to which 
are affixed only six legs, proves that even this circum- 
stance possesses no weight when set against the organi- 
zation. If it was a difference in this respect, that proved 
the Crustacea classically distinct from Insecta—that like- 
wise was the principal reason for the separation also of 
the Arachnida —it seems to follow that it ought also to 
furnish an argument equally cogent for considering the 
Trachean Arachnida, as well as the Myriapoda, distinct 
from the Pulmonary. 
Another difference between the tribes in question is 
that of their metamorphosis ; and this appears to have 
had great weight with Lamarck, inducing him to include 
in his Arachnida, not only the Tracheans and Myriapods, 
but even the apterous Hexapods, except Pulex, or the 
Anoplura and Thysanura of modern authors. But the 
metamorphosis alone, unless supported by the internal 
organization, will I think scarcely be deemed a sufficient 
reason for separating from each other tribes agreeing in 
that respect, and placing them with others with which 
they disagree. ‘The metamorphosis in some of the Hex- 
apods (Lepidoptera) consists in the loss of legs, the ac- 
quisition of wings, a great change in the oral organs and 
in the general form; in others (some Coleoptera), in the 
acquisition only of wings and a change of shape, the oral 
organs remaining much the same; in others again (Cur- 
culio L.), in-the acquisition of six legs and wings and a 
change of form; in the flea, in the acquisition of six 
their head is confluent with the thorax; but in the East Indian 
one (G. fatalis), and in the Spanish one described by M. L. Dufour 
(G. intrepidus), it is distinct. 
* Dufour ubi supra. Hor, Entomolog. 382. 
