Lazy bugs of August

Insects deserve siestas, too.

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a punctured tiger beetle Cicindela punctulata — Photo by Matt Pelikan

Insect life often takes a siesta on the Vineyard in early August. In the hot, dry weather, plants stop growing, so there is little high-quality food for herbivorous insects to eat. And all insects, herbivorous or predatory, face the risk of dehydration during a long spell of hot, dry weather. The bugs are out there, for sure, but their lives are largely on hold, activity limited and growth postponed until wetter weather prompts a new round of feeding and development.

In looking for insects to watch under these conditions, I sometimes apply a counterintuitive rule: When conditions get harsh, seek out the places where they’re harshest. Insects adapted to life on sterile, barren soil don’t seem to notice drought very much, because their preferred conditions already resemble a drought-stricken landscape.

So last Friday, I livened up a generally unbuggy day with a visit to an area in Edgartown that features

patches of gravelly, nearly plantless soil. Though a thin overcast filtered the sun, the ground was hot and crusted — about as inhospitable a site as one could ask for. And I was not disappointed, coming across an interesting insect drama playing out that left me, in equal measure, puzzled and amazed.

The first thing I noticed was the tiger beetle Cicindela punctulata, sometimes known as the punctured tiger beetle (for tiny pores that perforate its wing covers) or the sidewalk tiger beetle (for its preference for bare, hot, compacted surfaces). Predatory like all tiger beetles, and equipped with particularly long legs that hold its body well above hot surfaces, C. punctulata hunts mainly by sight, and simply runs down its prey with abrupt, dazzling sprints — the half-inch-long cheetah of the insect world.

The next thing I noticed were dozens of tiger beetle larval burrows. The larvae of tiger beetles live in tunnels, ambushing prey items that pass close to their holes with powerful jaws, and then pupating underground before emerging as adults. The mouths of these tunnels are quite distinctive: neat, precisely circular, about an eighth of an inch in diameter, plus or minus, depending on the age of the occupant.

The third thing I noticed were plump, mostly black flies cruising in loose S curves about a foot above the ground. I managed a few photos, and later identified these flies as Anthrax georgicus, a common and widespread bee fly that, like most insects, has no common name.

Anthrax georgicus is known to be a parasite on tiger beetle larvae, laying eggs in the larval burrows of the beetles; when the eggs hatch, they feed on the beetle larva, eventually killing it before a mature fly emerges to repeat the cycle. But I had never seen a bee fly laying eggs, so I watched intently.

When its simple but effective search algorithm brought it over a tiger beetle burrow, a fly would instantly hover in place, clearly recognizing the target. Then it would make a series of about a half-dozen short downward swoops toward the burrow, presumably squeezing off an egg, aimed at the mouth of the tunnel, with each swoop. Dive-bombing tactics: So that’s how they do it! An egg-laying method that keeps the fly safely out of reach of the beetle larva’s jaws.

Still more surprisingly, the occupant of each bombed burrow appeared briefly at the tunnel’s entrance as the process ended. Perhaps they have an instinctive awareness of the threat the fly poses, and emerged to reject the eggs. Or perhaps something — the activity of the fly? The sound of tiny eggs pattering down around the burrow? — sounded like prey and lured the beetle larvae to the surface, making it more likely that a parasitic fly egg would stick to the beetle’s body.

The larvae seemed large for punctured tiger beetle larvae, which should still be young and quite small in early August. So the burrows may have belonged to another tiger beetle species, one of several that share the austere habitat of C. punctulata but are active as adults in spring and fall, rather than during the summer. I’d need to get a specimen under a microscope to ID it for sure (tiger beetle larvae are distinguished by fine details of structure, such as the number of hairs growing on various body parts).

It’s very possible that larvae of two or more tiger beetle species were present, perhaps at different stages of development, and now I wish I had paid more attention to the size and appearance of the burrows. And I’ll need a lot more time watching them before I have any real sense of how Anthrax recognizes its targets and achieves its lethal goal.

So, as always it seems, the experience left me with more questions than answers. But I enjoyed the opportunity to watch such a complex interaction among species, and to see how even in the most unpromising settings, some creatures feel perfectly at home.