Common spider tortoise
The carapace (to 15 cm) is rounded dorsally. Each dark brown or black vertebral and pleural has a star-like pattern consisting of a yellow center from which extend several rather broad rays (usually 6-8 on each vertebral, 4-6 on each pleural). The cervical scute is often absent, but when present is long and narrow. The neural bones are either octagonal or squared. The plastron is hinged allowing the forelobe to be raised to almost close the anterior portion of the shell. The hindlobe is posteriorly notched. The plastral formula is abd > gul > pect > hum >< an > fem. Plastral pattern varies from totally yellow to having some dark blotches at the bridge. The maxillae lack a median ridge, but the sides of the upper jaws are serrated. The quadrate does not enclose the stapes, and the anterior palatine foramina are large. The head is black with some yellow speckles. Neck vertebrae are odd in being either totally procoelous cervical vertebrae or with the second vertebra biconvex, only a rare anomaly in other turtles (Williams, 1950a). Limbs and tail are yellowish brown. The tail is not noticeably flattened.
Males have longer, thicker tails with a more developed terminal spine. Females are, on average, larger than males; mean carapace length 12.2 cm in females, 11.6 cm in males (Jesu and Schimmenti, 1996).
Pyxis arachnoides occurs only along the southern coast of Madagascar from the Mahajamba River southward around Cape Sainte-Marie almost to Fort-Dauphin.
Three subspecies are currently recognized. Pyxis arachnoides arachnoides Bell, 1827, the common spider tortoise, occurs along the southwestern coast of Madagascar in the vicinity of the Onilahy River, which virtually splits its range into northern and southern portions. Its uniformly yellow plastron has a movable hinge, forward projecting gulars, and axillary scutes only a little longer than broad when viewed from beneath. P. a. oblonga Gray, 1869a, the southern spider tortoise, ranges along the southern coast of Madagascar from La Linta to Lake Anony. Its plastron has a movable anterior hinge, some black pigment or blotches, and only slightly projecting gulars. Its axillary scutes are broader than long when viewed from below. The northern spider tortoise P. a. brygooi (Vuillemin and Domergue, 1972) occurs in the northern portion of the range, southwest of the Mangoky River between Morombe, Lake Ihotry, and Fanemotra Bay. It lacks mobility in its anterior plastral hinge, has some black pigment on the plastron, slightly projecting gulars, and axillary scutes longer than broad when viewed from beneath.
The spider tortoise lives in tropical deciduous forests and dry woodlands (Auffenberg and Iverson, 1979).
Practically nothing has been reported on the life style of this tortoise. According to Bour (1981a) only a single egg is laid per clutch, and this egg is similar in dimensions to that of P. planicauda.
IUCN Red List Status (1996)
Vulnerable (B1+2abcd). The Malagasy spider tortoise has declined in numbers over the last few decades, and recruitment of young may be low in some areas. Jesu and Schimmenti (1996) found the age classes hatchling to seven years to be practically absent in a sample along the Onilahy River, even though the density there was relatively high, about 3 tortoises per hectare, and thought this most likely due to local collecting of young individuals for the pet trade and predation on eggs and hatchlings by introduced wild boars (Potamochoerus larvatus).