A raneae > 
91 
Order 4. — Araneae (Spiders). 
The carapace of the cephalothorax (prosoma) is unsegmented Table- 
in the spiders, and the eyes are situated in the middle of its ^^g^ 08 ' 
anterior margin ; they are usually eight in number, and are 
typically arranged in two transverse rows, but there are many 
other arrangements in the various families. The first appendages 
or chelicerae consist of two segments, the basal one of which 
contains the poison-gland, whilst the apical one forms a retroverted 
fang. All the remaining appendages are leg-like in form ; in the 
male a complicated copulatory organ is present on the lower side 
of the terminal segment of the second appendage or palp. A narrow 
pedicel separates the cephalothorax from the “ abdomen. ” (opis- 
thosoma) ; with very few exceptions the latter is unsegmented, and 
its lower surface is always furnished with a number of spinning 
appendages. Two pairs of lung-sacs may be present, but the 
posterior pair of these is replaced by tracheal tubes in most spiders, 
and in a few 7 species this is also the case with the anterior pair. 
Sound-producing organs, which are sometimes very complex in 
structure, occur in a large number of Mygalomorph spiders. They 
usually consist of arrangements of spines and rods which are 
situated on the opposed surfaces of the basal joints of the anterior 
limbs (either between the two chelicerae, or between the chelicerae 
and the palps, or between the palps and the legs of the first pair). 
The presence of a stridulatory organ in these bird- eating spiders 
was first made known by Professor Wood-Mason in an Assamese 
species ( Chilobrachys stridulans). In this spider the inner surface 
of the basal segment of the palp is furnished with a row of vibratile 
bacilliform bristles and the opposed surface of the chelicera with a 
number of strong spines. When irritated the spider assumes 
a threatening attitude, raising itself upon its hind legs and 
brandishing the front legs in the air, at the same time making an 
audible rasping noise by rubbing together the basal segments of 
the two anterior appendages. 
Another very similar type of stridulatory apparatus is present 
in a number of the Arachnomorph spiders of the family Sicariidae 
(in the genera Sicarius and Scytodes). The inner surface of the 
femur of the palp in these spiders bears a single tubercle (or a 
longitudinal row of tubercles), whilst the outer surface of the 
chelicera is provided with a series of well-marked transverse ridges. 
The noise made by the spiders of this family has been compared to 
the buzzing of a bee. 
