A mature St Andrew’s Cross spider (Argiope keyserlingi) has set up her web between two maidenhair ferns on our back patio and I noticed yesterday morning that she had caught and wrapped a substantial meal, perhaps a fly or a small moth.
Looking more closely after lunch (my lunch, that is, not hers), I saw a much smaller spider hanging around in the edge of her web.
The macro lens with a close-up filter was able to show that it was a Dewdrop Spider, Argyrodes antipodianus, and even that it was a male (those “boxing gloves”, really enlarged palps, are the giveaway). St Andrews Cross females grow to 16 mm according to Arachne.org while Dewdrop males grow to 2.5 mm according to the same site, and both were close to those sizes.
Dewdrops and others in the same genus are kleptoparasites, living in the webs of much larger spiders and stealing from them. It’s a surprisingly widespread lifestyle (see Wikipedia for examples), but its dangers are obvious: the host might catch and eat the freeloader. In this case, we think that the Dewdrop is so light-footed that the St Andrew’s Cross doesn’t respond as it would to the struggles of worthwhile prey blundering into her web.
As I watched, the Dewdrop spider circled around his host, who had finished her meal and was preoccupied with unwrapping her prey before discarding the dry remains. Eventually, after timid approaches and panicky retreats, Dewdrop laid his fangs on her meal.
Whether he extracted any nutrition from it is doubtful, but at least he survived the attempt.
A little while later he was in the lower section of her web with fresh prey which was his to enjoy in peace. It had been caught in her web but it was far too small for her to bother with. This, of course is the other (and probably safer) way in which Dewdrop spiders benefit from their host’s work.
