DEFINITION OF THE TERM INSECT. 93 
approach to such structure may be traced in some Hexa- 
pods; for instance, the coalition of the head and trunk 
in Melophagus, Latr., and that of the trunk and abdomen 
in Sminthurus, Latr.2 The Myriapoda exhibit other re- 
markable differences; though their head and trunk are 
distinct, the former antenniferous, and their body annu- 
lose, the abdomen as well as the trunk is furnished with 
legs, sometimes amounting to hundreds; but even to this 
a tendency has been observed in some Hexapods?. ° If 
you examine a specimen of Machilis polypoda, an insect 
related to the common sugar-louse (Lepisma saccharina), 
you will find that the abdomen is furnished with a double 
series of elastic appendages, which, being instruments of 
motion, may be regarded as representing legs. It is 
worthy of notice, that the Myriapoda when first disclosed 
from the egg have never more than szz legs‘, and keep 
acquiring additional pairs of them and additional seg- 
ments to their abdomen as they change their skins: and 
it is equally remarkable, that many Hexapods are subject 
to a law in some degree the very reverse of this, having 
many abdominal legs in their first state, and losing them 
all in their last. The union of the head with the trunk 
in the Trachean Arachnida has heen regarded as almost 
an unanswerable argument, in spite of their different in- 
ternal organization, for including them in the same class 
with the Pulmonary Arachnida ; but the case of Galeodes, 
which, though furnished with gills, (as an eminent Rus- 
sian Entemologist Dr. G. Fischer is reported to have 
discovered‘,) implying also a circulation, and evidently 
® De Geer, vii. ¢. iii. f. 8. ® Hor. Entomolog. 351. 
© De Geer, Ibid. 571, 583. ¢. xxxvi. f. 20, 21. 
aN. Dict. d Hist. Nat. xxvi. 445. Mr. W.S. MacLeay informs 
me that the West Indian Galeodes breathes by trachex, and that 
