8 SOUTHERN CALIFORNIA ACADEMY OF SCIENCES 
Some spiders and seorpions have the power of stridulation ; 
the stridulating organ being composed of spines and clubshaped 
hairs, situated, in the spiders, between the fangs and the base 
of the palps; in the scorpions, between the pincers and the 
basal joint of the first pair of legs. In the case of an African 
scorpion (Parabuthus), the animal possesses the power of—as 
it were—playing the mandolin; which it does by striking the 
point of the sting along prominent striations or ridges situated 
on the keels of the first and second caudal segments. This 
sound, like the rattle of the snake, serves to warn enemies of 
the dangerous nature of the animal which produces it. It has 
been contended that this stridulation is m the nature of a sex- 
call; but this is a disputed point, and is at least open to ques- 
tion; since the Arachnida have no organ of hearing, such as is 
situated in the antennule of the lobster; or the tympanum which 
in some crickets is placed in the foreleg, in others above the in- 
sertion of the wing. Yet it seems that spiders are to some 
extent sensitive to vibrations of air or substance. I have, by 
striking the pavement as far as 3 feet off the largest of the 
Californian spiders, Eurypelma (commonly but incorrectly 
called the tarantula), obtained a response in the shape of the 
assumption of a defensive attitude. Cteniza will run up her 
tube and hold down the trapdoor, if the outside of it is gently 
scratched. And Dr. McCook has caused spiders to respond to 
the vibrations of a tuning fork. It is likely that this perceptive 
sense, in the absence of an auditory organ, resides in the sete, 
or long specialized chitinous spines, seated in pits or depressions 
of the exo-skeleton. These differ in structure from true hairs, 
and are present, in greater or less degree, in all the orders 
undr consideration. These different protective devices are 
needed by the Arachnida, in spite of then formidable weapons; 
for they have various enemies more powerful than themselves. 
Chief of these are (apart—of course—from birds) various spe- 
cies of wasps and flies. In California, the most conspicuous of 
these, by reason of its size and brilliant coloration, is the *“‘tar- 
antula-hawk’’—Pepsis formosa—which attacks the Eurypelma, 
an dalso—I believe—the trap-door spider. Not the wasp, how- 
ever, but its larva, feeds on the victim, which is paralyzed but 
not killed. An egg is then deposited on or in its body, which 
is then buried. The larva hatches out in due time, and feeds 
on the living but helpless body of its unwilling host. J had an 
opportunity of observing an attack made by Pepsis on an Eury- 
pelma. The vigor and directness of the assault were remark- 
able; and the spider seemed to realize the hopelessness of re- 
sistance; for it huddled up helplessly, as does a rabbit when 
pursued by a ferret, and made no attempt to defend itself. 
