and are in constant motion, as if feeling for obstacles in the 
way (see Fig. 24). 
Formerly you were told to observe that in insects the body 
is divided into three parts — head, thorax and abdomen — and 
it is to be regretted that so soon you find an exception, for 
the body of the plant louse is one solid piece, pear-shaped, 
with head at the narrow end. However, you will observe 
as you continue to study this insect, that this is not the only 
exception which you will find in comparing it with a typical 
insect. You do find the six legs, which are very slender and 
comparatively long, each one terminated by a pair of minute 
hooks, which serve the insect to cling to plants. 
Now let me call your attention to a most remarkable fact in 
the life history of this little animal, a fact which was not dis¬ 
covered by naturalists until the year 1743, when a close study 
of the plant louse was made by a Swiss—Charles Bonnet—and 
the report of his discoveries created great excitement in scientific 
circles. It was he who discovered the fact that the aphides, 
the name by which these plant lice are known to entomologists, 
give birth to young resembling their parents in the possession 
of legs, antennae, eyes and proboscis. This, you see, is a re¬ 
markable exception to the rule in insect life, that the young are 
hatched from eggs, or that eggs are laid and the young hatched 
therefrom. So you must understand now that this insect does 
not pass through the four stages which we assigned to insect 
life—that is, the egg, the larva, the pupa and the imago. Bonnet 
made his discoveries by placing a single insect as soon as it was 
born upon a suitable plant, and so enclosed it that it could not 
escape and so that no other insects could enter. In eight days 
he found that the young plant louse was full grown and had 
given birth to a young insect. Others were born on successive 
days, until there was a family of ninety young of this one mother. 
He took from this cage one of the young as soon as it was 
born and isolated it, when, again, after eight days, the young 
began to be born. He continued this experiment through five 
generations, when, through some accident, he was forced to 
terminate this interesting work. He showed that each individ¬ 
ual, under favorable circumstances, may become the mother of 
ninety children, and, by carrying the calculation of a possible 
progeny through several generations, he found that in the second 
4 
