DEFINITION OF THE TERM INSECT. 51 
retracted within the body. This part is the principal 
seat of the respiratory pores or spiracles, many having 
eight in each side, while others have only one. 
Such are the principal external characters which di- 
stinguish Insecta and Arachnida, or what we have here- 
tofore regarded as insects, to which here may be added 
another connected with their internal organization. The 
union of the sexes takes place in the same manner as 
amongst larger animals; and the females with very few i 
exceptions, more apparent than real, are oviparous. 
They are, however, distinguished by this remarkable 
peculiarity already alluded to, that, except in the case of 
the Arachnida, one impregnation fertilizes all the eggs 
they are destined to produce. In most cases after these 
are laid, the females die immediately, and the males after 
they have performed their office, though they will some- 
times unite themselves to more than one female. One other 
circumstance may be named here—that no genuine insect 
or Arachnidan has yet been found to inhabit the ocean. 
Before I conclude this letter, it is necessary to apprize 
you, that every thing which it contains relative to the 
characters of insects, has reference to them only in their 
last or perfect state, not in those preparatory ones through 
which you are aware that the majority of them must pass. 
The peculiar characteristics of them in these states—in 
the egg, the Jarva, and the pupa, will be the subjects of 
my next letters, which will be devoted to a more detailed 
view of the metamorphosis of insects than I gave you 
before when adverting to this subject?. 
Iam, &c. 
* See above, Vor. I. Ed. 4, p. 63—. 
E 2 
