STRUCTURE AND ECONOMY OF SPIDERS. 297 
tion be extended to other retiary spiders, the feet of 
many species which construct complicated snares will 
likewise be found to exhibit a similar organization. 
As the best means of guarding against errors, to 
which the inspection of limbs defective in structure 
might conduce, it is advisable to select the legs of 
vigorous individuals which have recently oe 
whenever such can be procured. 
The supernumerary claws were first observed by 
me in examining the feet of Apetra apoclisa ; and in 
every instance I counted as many as five, which, with 
the three upper ones previously known, give a total 
of eight claws on the same foot, distinguishable at a 
glance from the coarse, setaceous hairs in their vici- 
nity. There is also a strong, movable spine inserted 
near the termination of the tarsus of each posterior 
leg on the underside, which curves a little upwards at 
its extremity, and exhibits a slight irregularity of out- 
line at its superior surface. The function performed 
by these spines, which have been named. sustentacula, 
is an important one. By the contraction of their 
flexor muscles they are drawn towards the foot, and 
are thus brought in immediate opposition to the 
claws, by which means the animal is enabled to hold 
with a firm grasp such limes as it has occasion to 
draw from the spinners with the feet of the hind 
legs, and such also as it designs to attach itself to. 
Now, as the sustentacula and the spinning-apparatus 
are the most efficient instruments employed by the 
