(a) Araneida (Spiders).
Almost all Arachnids possess poison-glands, which are connected, in some cases with the buccal apparatus, in others with a special inoculatory organ situated at the posterior extremity of the body. The spiders and scorpions belong to this group, and their venom is particularly active.
On each side of the mouth of spiders is found an appendage ending in a fang (chelicera), at the extremity of which opens the excretory duct of a more or less developed poison-gland. The venom produced by these glands is instantly fatal to all small animals upon which spiders feed. In man and large mammals their bite produces sensations of pain accompanied by swelling and muscular contractions as though caused by localised tetanus.
The venom of certain species of spiders sometimes causes very serious and even fatal results. Latrodectus malmignattus (the malmignatte of the South of France and Italy), and especially Latrodectus mactans, of Chile ([fig. 96]), are greatly dreaded.[111] The area of distribution of the latter includes the whole of Tropical and Sub-tropical America. It is said that it frequently causes the death of milch cows, and that in man its bite produces tetanic effects, which last for several days, but are in most cases amenable to treatment.
Fig. 96.—Lactrodectus mactans (formidabilis olim).
1, Female, twice natural size; 1a, its eyes, greatly enlarged.
Another dangerous spider is the Katipo (Latrodectus scelio), of New Zealand. This creature is confined to the sea-shore, and the natives are often bitten when collecting shell-fish or sea-weed. The Maoris are so much afraid of the bite of the Katipo that, when one of them has been bitten in his hut, and the animal cannot be found, they do not hesitate to burn the dwelling to the ground. Moreover, they are convinced that the death of the spider is absolutely necessary for the recovery of the patient.[112]
Kobert[113] has made an experimental study of the venom of species of Latrodectus and Epeira. That of Latrodectus erebus (the Karakurte of South Russia) is particularly toxic.
If a dry extract of these spiders be prepared and injected into the veins of dogs or cats, it is found that a few milligrammes per kilogramme are sufficient to cause death, with phenomena of dyspnœa, convulsions, and progressive paralysis of respiration and the heart. Rabbits, rats, birds, frogs, and leeches are also sensitive to this poison, though the hedgehog is almost refractory. The young spiders, and even the eggs, are more toxic than the adults.
Spider-venom is destroyed by heating for forty minutes at + 70° C., and is precipitated by alcohol. When absorbed by ingestion it has no effect: it is hæmolytic and coagulates blood.