A nematode-filled Bembidion canadianum

Shannon, a student in the Discovering Insect Species course, went down to Eugene last weekend and looked for Trepanedoris. To say that she did well would be an understatement.  She caught all four species that I had seen from the Willamette Valley of Oregon (Bembidion acutifrons, B. elizabethae, B. fortestriatum, and B. siticum), and more.

The real prize was a male of a species I had never seen from this area.  Here’s the live beetle:


At first glance I thought it was an undescribed species, but after looking at it more closely I now think that it is an extremely small specimen of Bembidion canadianum.  I have only ever seen one specimen of Bembidion canadianum from Oregon, collected in 1955 from east of the Cascades, around Upper Klamath Lake.  The specimen is housed in the OSAC.

The specimen Shannon caught was a male with a rather swollen abdomen, and it walked very slowly.  I’ve seen this before, and it usually points to some unpleasantness (at least for the beetle) inside its body cavity, in the form of nematodes.  And, sure enough, when I opened up the abdomen to expose the tissues to ethanol so that the DNA would be well-preserved, there were a lot in there:


Here’s what part of the abdomen looked like; all of the little white tubes here are nematodes:


I’ve seen many nematode-filled Bembidion over the years, but this one was unusual, in that it had nematodes of different sizes: a few big ones, and many small ones.  Normally all the nematodes are of more or less the same size. Another thing that is unusual about this beetle is that it still had what appeared to be fairly healthy testes and accessory glands; a nematode-filled Bembidion normally has very little of its own tissue remaining.

Although interesting, the nematodes will cause some problems in extracting beetle DNA from the specimen – as it will be hard to avoid extracting nematode DNA instead!

This entry was posted in Revising Bembidiina, Z499 (Discovering Insect Species) and tagged , , . Bookmark the permalink.

2 Responses to A nematode-filled Bembidion canadianum

  1. Pingback: Discovering Insect Species: Overview in Rearview | The Subulate Palpomere

  2. James Bergdahl says:

    Weekly over many years I used to process many thousands of carabid beetles I trapped in 4% formalin in the San Juan and Gulf Islands. I vividly remember a Pterostichus lama, one of the largest carabids in the coastal fauna, pickled with a VERY long nematode coming out of its anus, presumably an attempt to escape at the last minute. I figure many individual carabids in our area are loaded with parasites. Population densities of some carabid species on small islands in the San Juan and Gulf islands can be extremely high compared to large islands and the adjacent mainland. I suppose this is may be due to: 1) a marine subsidy of their food webs, 2) escape from parasites, and 3) escape from some primary predators (e.g. shrews), perhaps all three at the same time and place.

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