These experiments were continued on cats, dogs, guinea pigs and various other animals. Not only extracts from the cephalothorax, but from other parts of the body, from newly hatched spiders, and from the eggs were used and all showed a similar virulence. Every effort was made to avoid sources of error and the experiments, conducted by such a recognized authority in the field of toxicology, must be accepted as conclusively showing that this spider and, presumably, other species of the genus _Latrodectus_ against which the clinical evidence is quite parallel, possess a poison which paralyzes the heart and central nervous system, with or without preliminary stimulus of the motor center. If the quantity of the poison which comes into direct contact with the blood is large, there may occur hæmolysis and thrombosis of the vessels. On the other hand, check experiments were carried out, using similar extracts of many common European spiders of the genera _Tegenaria_, _Drassus_, _Agelena_, _Eucharia_ and _Argyroneta_, as well as the Russian tarantula, _Lycosa singoriensis_. In no other case was the effect on experimental animals comparable to the _Latrodectus_ extract. Kobert concludes that in its chemical nature the poison is neither an alkaloid, nor a glycoside, nor an acid, but a toxalbumen, or poisonous enzyme which is very similar to certain other animal poisons, notably that of the scorpion. [Illustration: 9. Latrodectus mactans; (_a_) female, x 3; (_b_) venter of female; (_c_) dorsum of male. After Comstock.] The genus _Latrodectus_ is represented in the United States by at least two species, _L. mactans_ and _L. geometricus_. Concerning _L. mactans_ there are very circumstantial accounts of serious injury and even death in man[B]. _Latrodectus mactans_ is coal black, marked with red or yellow or both. It has eight eyes, which are dissimilar in color and are distinctly in front of the middle of the thorax, the lateral eyes of each side widely separate. The tarsus of the fourth pair of legs has a number of curved setæ in a single series. It has on the ventral side of its abdomen an hour-glass shaped spot. The full-grown female is about half an inch in length. Its globose abdomen is usually marked with one or more red spots dorsally along the middle line. The male is about half as long but has in addition to the dorsal spots, four pairs of stripes along the sides. Immature females resemble the male in coloring (fig. 9). Regarding the distribution of _Latrodectus mactans_, Comstock states that: "Although it is essentially a Southern species, it occurs in Indiana, Ohio, Pennsylvania, New Hampshire, and doubtless other of the Northern States." _L. geometricus_ has been reported from California.