88 ANIMAL ACTIVITIES. 
tant organs. Devices for defence, for eluding enemies, 
and for procuring appropriate food among insects are 
everywhere seen to be varied modifi- 
cations of the same organ or organs. 
In general such structures are found 
in any particular insect as best help 
it to preserve its life inthe particular 
Fic. 82,—Fore Leg environment in which it lives. So 
of a Butterfly. much does the structure tell us about 
the mode of life, that we are often 
able to infer the habits of an insect which we have 
never seen alive from the study of dead specimens, and 
even from fossil remains. 
Changes of Organs Because of Changes of Habit. 
That these variations of similar organs have arisen 
gradually, through changes of habit made necessary 
by changes in surroundings, is generally believed. 
At first sight the long delicate proboscis of the butter- 
fly, the lapping tongue of the house-fly, the beak of 
the aphis, the hard, biting jaws of the beetle seem very 
different structures, but when we watch the caterpillar 
of the butterfly and the grub of the beetle with mouth- 
parts so much alike at the start, and see that in one 
case the maxillz elongate into the coiled proboscis, 
and in the other the mouth-parts grow into the formid- 
able and destructive biting-organs of a carnivorous 
beetle, we wonder less at the divergence than at the 
resemblance. We see, too, how it may have been 
possible for organs very unlike to have arisen from 
similar beginnings. The great variety seen in the 
breathing-organs of aquatic insects furnishes another 
illustration of the change of organs necessitated by a 
change of habit. From the fact that all aquatic insects 
breathe air by trachea at some period of their life it is 
believed that their ancestors, as well as the ancestors 
of insects having aquatic larve, were originally terres- 
trial. Either driven by enemies or lured by more 
abundant food, at some distant period, these insects 
