ZOOLOGY. 605 



seaicliiiig for fossils iu the Siluriau rocks, aud especially for terrestrial 

 types, will be prosecuted, for such discoveries will serve as very efficient 

 stimulants. {Nature, V. 31, pp. 295-298.) 



Curious mimicry hy a Spider. — A curious case of mimicry by a spider 

 has been recorded by Mr. H. O. Forbes. The spider in question is found 

 iu Sumatra, and has been named Thomisus decipiens. On June 25, 1885, 

 iu a forest of Sumatra, Mr. Forbes' attention was excited by his " eyes 

 resting on a bird-excreta -marked leaf." On examination it was fouud 

 that the appearance was deceptive and had been i)roduced by a spider 

 which had so closely copied nature that the imitation would readily 

 deceive the uncritical observer. "The spider is in general color white, 

 spotted here and there with black ; on the under side its rather irregu- 

 larly shaped and prominent abdomen is almost all white — of a pure 

 chalk white; the angles of the legs are, however, shining jet black. 

 The spider does not make an ordinary web, but only the thinnest film 

 on the surface of the leaf. The appearance of the excreta rather 

 recently left by a bird on a leaf is well known. There is a pure white 

 deposit in the center, thinning out round the margin, while in the 

 central mass are dark portions variously disposed ; as the leaf is rarely 

 horizontal, the more liquid portions run for some distance. Now, this 

 spider, one might almost imagine to iiave in its rambles marked and 

 inwardly discerned what it had observed, and had set about practicing 

 the wrinkles gained -, for it first weaves a small irregular patch of white 

 web on some prominent leaf, then a narrow streak laid down towards 

 its sloping margin, ending in a small knob. It then takes its place on 

 the center of the irregular spot on its back, crosses its black-angled 

 legs over its thorax and waits. Its pure white abdomen represents the 

 central mass of the bird's excreta, the black legs the dark portions of 

 the slime, while the -web above described represents the more watery 

 marginal part (become dry), even to the run-ofl' portion with the thick- 

 ened knob (which was not accidental, as it occurred in both cases), like 

 the residue which semi-fluid substances, ending in a drop, leave on 

 evaporation. It keeps itself in jjosition on its back by thrusting under 

 the web below it the spines with which the anterior ui)per surface of 

 the legs is furnished." 



The most interesting fact of all, in the oi)iniou of Mr. Forbes, is " not 

 so much that of the spider having gaii.ed, which it can, of conrse, have 

 no consciousness of, by natural selection, the color and form of an ex- 

 cre turn, but that it has acquired the habit of supplementing its own 

 color and form by an addition in such absolute harmony with that of 

 which itself is the similitude." 



This species, on being reexamined by the l^ev. O. P. Cambridge, the 

 distinguished arachnologist, was con.sidered to represent a peculiar 

 genus which was named Ornithoacatoidbs, from its simulation of a bird's 

 excreta, Further, Mr. Cambridge recognized in various collections 



