Beck_, on the Reproduction of an Acarus. 
31 
and many subsequent generations have been produced by 
them. I am now able to supply the following facts connected 
with this acaruS;, which I believe are new. 
The eggs as compared with those laid by other species of 
acari with which I am acquainted are rather small in propor- 
tion to the parent ; they are of a bluish-white colour^ trans- 
parent^ and adhere to the substance they are laid upon by a 
short thread at one extremity. At the age of two or three 
days in summer time^ the young may easily be detected in- ^ 
side the egg, which hatches according to my memoranda in 
five,, six, or seven days from the time it was laid ; the varia- 
tion in this and other periods of development being due I 
believe in great measure to the temperature of the atmosphere. 
The young as it comes from the egg has only six legs, it is 
white, perfectly transparent, and very active, wandering about 
in every direction. At the age of seven days it casts a skin, 
and then acquires eight legs, at a further interval of seven or 
eight days it casts a second skin, and then arrives at maturity ; 
before each of these moultings the individual remains some- 
times for one or two days perfectly stationary and apparently 
dead ; I mention this circumstance so that any one who likes 
to repeat these experiments may not disturb the acarus in this 
important operation. 
This acarus soon after arriving at maturity assumes a 
yellowish-green colour, and I will endeavour to describe some 
of its more remarkable features at this stage of its life. 
That which strikes one at first sight is the size of the falces, 
for I presume they cannot be correctly termed mandibles; 
they are largely developed, apparently very powerful and 
move in a horizontal direction; the two when spread out 
forming a complete semicircle. The free extremities of the 
falces are somewhat complicated in structure ; on the outside 
edge is a strong claw, with two short spurs at its base, and 
immediately within this or the inner side are two combs, very 
similar in general appearance to the pectinated claws at the 
extremities of some spiders^ feet; the inner one is smaller 
than the other, but the two move simultaneously and inde- 
pendently of the outer claw. There are also a few strong 
hairs situated near the combs. 
When this acarus seizes another one of a different species, 
which it does by its falces, laying hold of a leg or any other 
part indiscriminately ; the prey after a lapse of about fifteen 
or twenty seconds becomes poisoned or paralysed, the legs 
bend up under the thorax, and no part of its body makes any 
resistance to the pulling backwards of the devourer, who, 
when she finds this passive condition of her prey , deliberately 
