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W. RAE SHERRIFFS
THORAX.
The
Behind the head area of the cephalothorax follows the thoracic part, often marked dorsally by a distinct median groove (fovea) or pit, bearing laterally four pairs of legs and ventrally the stout breast plate (sternum).
There is no easily made out mouth for the food is liquid. mouth lies at the base of the pedipalps. The opening which is purely suck- ing is bounded by small hard plates acting as lips. The flesh of the victim is lacerated and held close to the mouth by the mouths parts and the juices then sucked inside.
Each spider leg consists of seven joints, coxa, trochanter, femur, patella, tibia, metatarsus and tarsus which end in two or three claws which are sometimes beautifully fashioned like tiny combs with minute teeth. It is these delicate instruments that enable the owner to hold on so easily to her silken web and also to grasp and manipulate the threads as she spins them in making her web. The relative length of the four legs is also im- portant in classification. The formula 1423 means that the first pair of legs is the largest and the second the shortest. To-day systematists lay great stress on the number and arrangement of the small spines found on the different joints of the legs. A special formula is employed for them.
ABDOMEN.
This second main piece of the spider's body is frequently the larger and dorsally is often marked by some pretty pattern, The shape varies enormously from globular to almost bricklike. Ventrally some very im- portant openings occur. Since there is no segmentation the abdomen differs entirely from that of the insect and there are no definite transverse rings. About one-third down its length the abdomen has across it in line three distinct parts, viz:—a central female genital opening, more or less covered and concealed by a genital plate (epigyne), and, on either side of this, the openings (stigmata) leading to the breathing organs ("lung-books"). Though usually thus named the respiratory organs are better styled "book- lungs" for they are breathing organs and certainly not books! The book- lung area may be sometimes lighter in colour than the surrounding parts because of the enclosed air.
About another third down the median line of the abdomen there may be present a single opening, the tracheal opening (spiracle) leading to branching respiratory tubes (tracheae) so characteristic of the insect. All true spiders have as breathing organs both book-lungs and tracheae, one pair of each.
Also in the middle line and occupying roughly the final third of the abdomen comes the bunch of spinning organs (spinnerets) through small holes in which the liquid silk issues from the silk glands within.
These spinnerets are found only in spiders. In fact the word spider means the spinner. These spinnerets are modified abdominal limbs and usually num- ber 6, three pairs termed fore, mid and hind.
All spiders both male and female produce silk from their silk glands through the spinnerets. Though all spiders yield silk, not all spiders make
The Hong Kong Naturalist.
The External Anatomy of the Spider
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webs with their silk. Different kinds of silk are manufactured and sent forth as a liquid that dries very quickly in contact with the air. One kind of silk is employed in constructing the main lines of the web. These radiate from the centre of an orb web and are not sticky. A second kind of silk is used for the viscid spiral lines of the web on which the flies stick fast. Yet still another kind of silk forms the cocoon which the mother spider weaves round the egg-mass to protect the contained ova.
Curiously enough the biggest and most conspicuous spinnerets are not found on those spiders which spin the most beautiful snares, Lastly behind the spinnerets, also in the middle line of the abdomen, is the anal tubercle on which opens the hind end of the food canal.
Among the spiders sent me from Hong Kong by Dr. Herklots were many specimens of a large Nephila.* It is convenient therefore to take this very handsome spider as the type, by means of figures of which we can illustrate all the external features as enumerated in this paper, Figure 1. Nephila is a veritable giant among spiders for when full grown her body measures about 5 cm. long and 2 cm. thick. Her leg span is some 15 cm. Her great web of strong yellow silk is well known in jungles during the rains when the spider reaches maturity. Nephila shows very beautifully sexual dimorphism in spiders, for the male is quite a dwarf compared with his formidable mate who is at least one hundred times his weight! In fact
we can roughly compare the relative sizes of the sexes here to a man six feet high mated to a wife as big as a church steeple! The so called dwarf or pigmy male contrasts strongly both in size and in colour with his relatively enormous consort for he is a minute reddish brown slender spider with con- spicuous palpal organs. He is commonly seen,-living at a respectful dis- tance from his gigantic partner on the outer edge of the great web at the centre of which the female reigns.
1.
PED
AP
SP
+
Figure 2. Spider
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Harvestman.
2.
Mite,
3.
After our description of the external features of a spider there na- turally arises the question “What is a spider?" In the old days--when
Nephila maculata. Fabr.
December 1932.