"So nat'ralists observe, a flea
Hath smaller fleas that on him prey;
And these have smaller fleas to bite 'em.
And so proceeds Ad infinitum."
Then, in the 19th century, mathematician Augustus De Morgan updated this to the more familiar
"Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum."
Well, this might not proceed quite ad infinitum, but there certainly can be several tiers of parasitism. One common example involves the small wasp Apanteles glomeratus (family Braconidae) which parasitises larvae of the 'cabbage' white butterflies Pieris brassicae and P. rapae. Eggs are laid into the butterfly larvae, where they hatch and develop - after about 15 to 20 days the wasp larvae emerge and the host dies. The wasps then pupate in silken cocoons next to the dead host, creating the arrangement that can be seen attached to a wide range of substrates, sometimes in very exposed situations (such as the middle of a glass patio door...), with the whole cluster attached by silk threads.
|A typical arrangement of Apanteles glomeratus cocoons by the dead host larva of Pieris brassicae.|
|Apanteles glomeratus cocoons showing the tangle of silk threads.|
I have seen numerous examples of this arrangement over recent days, largely because following several very poor summers in the UK, Pieris spp. are some of the few butterfly species to have done well, and consequently there are many of their larvae for parasites to target. Also, as Pieris spp. are ravenous eaters of brassicas in farms and gardens, A. glomeratus is a vegetable-grower's friend. However, if I look more closely, I might find 'hyperparasites' (parasites of parasites) such as the tiny ichneumon wasp Lysibia nana which targets braconids such as A. glomeratus. Closer still and - well, I don't think anyone has investigated - but I'd expect to find microscopic protozoan, fungal or nematode parasites. Maybe I need to capture some L. nana and have a look.
If you would like more technical detail about parasitic wasps, Ronquist (1999) gives details of their phylogeny, classification and evolution (including, in Table 1, some examples of who parasitises who). In contrast, if you fancy an excellent (and silly) photo of a braconid larva, try here instead (or, as well - I like both, and Myrmecos is a fine blog looking at entomology, often ants, from a more photographic viewpoint).