In the forests of Singapore lives a spider that must be an arachnophobe’s worst nightmare. Most species are solitary hunters subdue their prey with venomous fangs, sticky silken webs or a combination of the two. But Scytodes uses a third trick – it spits a sticky, venomous fluid from its fangs that both traps its victims and poisons them (see video of related species). And it does this in packs – after hatching, spiderlings spend their early lives on their home web and they spit at, bite and devour prey en masse.
There are actually about 200 species of spitting spiders belonging to the genus Scytodes, and the specific species I’m talking about here was previously classified as Scytodes pallida. But Laura Yap, a student from the National University of Singapore, believes that it may be a new species entirely. For the moment, she refers to it simply as “Scytodes sp“, and she has provided the first thorough description of its behaviour.
But Scytodes also has a tender side, with mothers caring for their spiderlings before and after hatching. By collecting colonies throughout Singapore, Yap found that the vast majority consisted of either single adults or a mother and her young brood. After laying her eggs in a silken case, the mother carries them around in two of her limbs until they hatch (see below). That process depends on her, for the babies can’t break out of the tough egg sac unless he makes a cut in it first. Once they’ve emerged, they stay with her for several weeks, and finally leave the web just before they mature.
On their own, the spiderlings are entirely capable of spinning webs and killing prey, but they suppress their hunting instincts in mum’s presence to let her do the work. She traps flies with her venomous spit and either leaves them on the web or drags them back to it. There, the spiderlings consume the meal together, often with mum at the table too. She never fights her young for food and she never spits at them.
The spiderlings themselves aren’t quite so selfless. They’ll happily share food that mum brings over, and once they reach a month in age, they will cooperate in groups of four or more to bring down flies much larger than themselves. But anything small that they kill on their own belongs to them, and they defend it accordingly.
The older they get, the more their cooperative streak fades. They become more likely to leave home, but also more likely to try their luck at cannabilism. The babies will turn on each other even if their mother is around and even if they have plentiful other sources of food. Indeed, the main cause of death among the spiderlings is each other. After six weeks, about a quarter of the babies have perished at the fangs of their siblings.
While all Scytodes species spit, only a few do so cooperatively. In fact, only about 40 of the 40,000 or so species of spiders are truly sociable. The species that Yap studied is a “subsocial” one, meaning that it hasn’t reached the same social state as ants, bees and termites, with their large colonies and castes of workers. They only form colonies because youngsters stay with their mothers after hatching.
It’s thought that this type of behaviour can evolve into the truly social lifestyles, akin to those of ants, by mothers caring for older and older offspring. Other species of social spitting spiders should provide a good testing ground for this idea.
Reference: Journal of Zoology doi:10.1111/j.1469-7998.2009.00555.x