Why is the cossus so destructive to the trees on which it lives?

Because it exists as a caterpillar three years previous to its changing into the pupa state. It is so tenacious of life, that it will remain in an artificial vacuum for hours together, without being injured; and under water for almost three weeks in the middle of summer. It is remarkable also, that the pupa has the power of locomotion, and when the time of its change approaches, can bore its way from the middle of the tree to the bark. - Blumenbach.

Why has the caterpillar of the cossus been stated by entomologists as a specimen of the muscular strength of insects?

Because it contains more than seven times as many muscles as the human body, being 4061. Lyonnet has ably illustrated the anatomy of this extraordinary creature. Mr. Rennie put one of these caterpillars under a bell-glass, which weighed nearly half-a-pound, and upwards of ten times the weight of the insect, yet it raised it up with the utmost ease. He then placed on the glass a thick book, weighing four pounds, and the animal again raised the glass, though loaded with the book, nearly a hundred times its own weight, and made good its exit. It should, however, be mentioned, first, that the wedge-like form of the caterpillar's head, in connexion with the peculiar shape of the glass, enabled it to lift it; and, secondly, that one side of the glass resting on the table, the insect only bore half the weight of the glass and book. - Insect Transformations.