Wednesday, 1 October 2008

Gap in the rain

During my searches for specimens to photograph, I begin to notice subtle things that would normally not be seen. A number of fly and moth larvae live inside the seedheads of flowers such as Knapweed, or Hardheads. This is a good place to hide, since there is a ready supply of food and the larvae are protected from the weather and from most predators or parasites. Notice the word 'most': I have found carnivorous beetle nymphs inside these seedheads and, of course, the relevant ichneumonids are equipped with long ovipositors that can reach inside the deepest seedheads. I like to find out which larvae are living inside the seedheads, and the best way to find out is to open them. But, of course, I have lots of failures: many seedheads are empty.

I have noticed, though, that there appears to be an external sign of the presence of some of the inhabitants. The first picture shows a 'normal' seedhead. Notice the shape and thickness of the stem as it approaches the seedhead:

This is another seedhead, where the stem is noticeably thicker (to my eyes). I know the difference is subtle, but I can detect it very clearly in the field:

Experiments have shown that the thicker ones are usually occupied, and the thinner ones are not, so I broke open the two shown above. The thin one was unoccupied, and the thicker one had a few larvae of Tephritid (Picture wing) Flies. Not conclusive, but it's just a trend, and it saves me from bursting open dozens of potentially empty seedheads. Of course, this makes me wonder why this should be, and I suppose I wouldn't be too surprised to find that the fly larvae have some effect on the formation of the seedhead which is beneficial to them. Perhaps something like making the seeds slightly further apart, to make movement easier?

It's seed time, and I wonder if anyone else sees skeletal hands in these empty seedheads of Cow Parsley.

My survey and study of the Nematus pavidus sawfly larvae continues. This almost-mature larva is moving between leaves, having polished off the lower one. The sun was directly behind the shot, hence the glare.

Incidentally, the picture shows one absolute and one subjective feature that separate sawfly larvae from lepidopterous (moth and butterfly) ones. The number of prolegs (the stumpy ones at the rear) is 6 (the anal claspers count, too). So this is certainly a sawfly larva, since 6 or more prolegs are indicative of the sawfly. Subjectively, I can see that the larva has a round-shouldered appearance: another feature of many sawfly larvae.

Does anyone else remember these?

1 comment:

Gill said...

"Does anyone else remember these?" Yep - but I haven't seen any for many a long year.

It's been such a wet late summer here I was beginning to think they'd never get all the harvest in, but a dry, even sunny, spell in mid-late September means they have - just - although I fear the quality of some of the crop won't be A1 - at least some wheat was sprouting in the ear!