Their inedibility, which they owe to a more or less coriaceous epidermis and an armature of strong sharp spines ([Fig. 6]).
Their brilliant colors—glistening black and white, yellow, fiery gold, metallic silver, rose-color, blue, orange and blood-red.
Their habit of hanging always exposed in the centre of the web.
Fig. 6.—Gasteracantha Crepidophora
(from Cambridge).
In an interesting discussion of the protective value of color and marking in insects, Poulton says that “the smaller convergent groups of nauseous insects often present us with ideally perfect types of warning patterns and colors—simple, crude, strongly contrasted—everything subordinated to the paramount necessity of becoming conspicuous,” the memory of enemies being thus strongly appealed to.
This proposition is well illustrated by the Gasteracanthidæ. Among larvæ the warning colors are almost invariably black and white, or black (or some very dark color), in contrast with yellow, orange and red. These are the colors that also constantly recur among the Gasteracanthidæ.
Cases that may be more justly considered exceptions to the rule that these hard, uneatable spiders are conspicuous are such species as Acrosoma rugosa ([Fig. 7]). One of this species was sent me by Mrs. Treat last summer. It lived for several weeks in my window, making no regular web, but hanging among a few irregular strands. It ate nothing, although provided with insects, but drank greedily of water. It might seem that its black and white coloring would make it conspicuous, but in connection with its irregular shape and its way of hanging motionless in the web it had the opposite effect.
We have no reason to suppose that the class represented in rugosa is like that touched upon by Poulton, in which very protectively colored larvæ suddenly assume a terrifying aspect on the near approach of an enemy; still they do enjoy a kind of double protection.