Spider-ectoparasitoids belonging to Ichneumonidae are known to manipulate web-building behaviour of their host spiders at the larval stage of penultimate instar. Their host specificity is quite high, whilst a wide range of spider groups are utilized as host by the whole group. Original web structures are thus diverse, and the mode of manipulation has also become diverse in response to each host web structure. To take a view of trends in evolution of web manipulation, accumulation of anecdotal studies is essential.
In this study, targeting a spider-ectoparasitoid (Zatypota albicoxa) utilizing a cobweb-weaving theridiid house spider, we’ve investigated its mode of manipulation in detail. The house spider constructs a three-dimensional web with numerous vertical snare threads downwards. These snares have sticky droplets at their distal end, called “gumfoot”, to capture and fish up wondering organisms. By taking pictures of 29 webs of parasitized spiders every day, we found that the number of gumfoot threads on the day at which the larva killed the spider (20.8 on average) is significantly (< 0.01) less than that of four each day before manipulation occurred (about 40-50). We’ve hypothesized that reduction of gumfoot threads has an effect to reduce a risk of web destruction by crawling organisms inadvertently bumping into the gumfoot snaring area no longer with spiders.