1988] 
Sierwald — Thalassius spinosissmus 
245 
Daily activity pattern 
During the summer season, spiders of all instars spend apparently 
most of the day on the water surface, unless hindered by heavy rain 
and storm (observations at study site between 9 am and 1 1 pm). 
Spiders kept in aquaria under a tent, exposed to daily temperature 
fluctuations demonstrated similar activity patterns. They were 
found in the hunting position nearly 24 hours a day and accepted 
prey at any hour (fed with tadpoles of Bufo rangeri and fruit flies). 
Hunting position 
In the hunting position, the spider anchors itself by one or two 
hind legs to a stone, log or vegetation. The other legs are extended, 
with metatarsi and tarsi touching the water surface (Fig. 1). During 
observation periods of up to two hours at the study site, the 
observed spiders did not change their location significantly unless 
they attacked prey or were disturbed. Single legs were frequently 
groomed with the chelicerae. 
Animals “resting” on the vegetation near the water often moved 
to the surface and assumed the hunting position in response to 
minor disturbances, a behavior observed frequently both in the hab- 
itat and in the laboratory, presumably serving camouflage. The 
white lateral stripes of the body blend well with the vegetation, 
disrupting the visual body profile. 
Diving 
If disturbed by touch or strong, close movement, T. spinosissimus 
submerges by climbing along the substrate. They are not able to 
break freely through the water tension nor can they swim under 
water. The spiders remained submerged for an average of five to ten 
minutes, although longer periods of up to 35 minutes were recorded. 
Prey capture 
T. spinosissimus hunts for appropriately sized prey (insects, 
crustaceans, fish, tadpoles, and toads) exclusively on the water sur- 
face. In the laboratory and in the field, prey was put on the water 
surface in different distances and positions from the spider. To cap- 
ture mobile prey, e.g., swimming water insects or tadpoles, the spi- 
ders waited until the prey had come close enough to be grasped with 
the front legs. A few spiders continuously orientated the spread 
front legs towards the moving prey. 
Prey struggling on the water surface such as terrestrial insects 
