SYSTEM OF INSECTS. 375 
ideas on this subject in a former letter, I need not here 
repeat what I then said a . 
iii. As we have subkingdoms, so we may also have sub- 
classesy or such large divisions of a class — not founded up- 
on internal organization or any of the primary vital func- 
tions, but upon different modes of taking their food, or 
such other secondary characters — as include more than 
one Order. To this description Clairville's Mandibidata 
and Hausiellata appear to me to belong, which I think 
are by no means entitled to the rank of Classes; for 
whoever compares these two tribes together will at the 
first glance be convinced, by the numerous characters 
they possess in common, notwithstanding the different 
mode in which they take their food, that they form one 
connected primary group. This circumstance, therefore, 
only furnishes a clue for their further subdivision into 
two secondary groups, separated by distinctions certainly 
of a lower value than those which separate the Crustacea 
and Arachnida from Insecta. This is further confirmed 
by the variations that take place in their mode of feed- 
ing in their different states ; some from masticators be- 
coming suctorious (Lepidoptera), and others from being 
suctorious becoming masticators (Mgrmeleon, Dytiscus, 
&c), — which shows that this character does not enter the 
essential idea of the animal. 
iv. Next to Classes and Subclasses we are to consider 
those groups of insects that are denominated Orders. 
The characters of these at first were taken principally 
from the instruments of flight or the absence of them ; 
and the name appropriated to each Order by Linne, after 
3 Vol. III. Letter XXVIII. 
