WATCH OUT, IT STINGS—OR DOES IT?

We all know when yellowjacket season rolls around, with pesky wasps that bother us on every picnic. You can usually chase them away easily, but they come back again and again. They are relatively innocuous when you’re trying to give them the brush-off, but don’t ever disturb one of their nests in the ground!

Western yellowjackets (Vespula pensylvanica) are members of the insect order Hymenoptera, the bees and wasps. Other wasps that are common in our area are bald-faced hornets (Dolichovespula maculata), bigger and fiercer than the yellowjackets and with a big turnip-shaped paper nest up in the trees; and introduced European paper wasps (Polistes dominula), with a much smaller paper nest with chambers open below.

All these species have in common a black and yellow or black and white banded abdomen. That coloration is common in wasps and bees and is thought to be aposematic (Greek ‘away sign’), a word used to denote a warning coloration. “Don’t mess with me” is a loose translation.

Some birds, for example kingbirds, are able to take stinging insects in their stride, catching them in flight and beating them to death, even squeezing out their stinger, but a lot of animals doubtless leave them alone because they pack such a punch at the end of their abdomen. The warning coloration assures that they are safe either because the predator species has a genetic memory of them or has tried to capture one previously and was stung by it, a much more immediate memory!

As these wasps move through our environment, relatively impervious to predation, other insects have benefitted by evolving coloration, shape, and even wing sound that mimic the wasps. Most of them in our area are flies (Diptera), especially hover flies of the family Syrphidae. Here are a few of them. They look very much like the wasps as they fly around, and even seasoned entomologists often have to look closely. I for one have grabbed what I thought was a fly from an insect net and been stung for my mistake.

By mimicking stinging wasps and bees, these so-called Batesian mimics (from Henry Bates, early student of this phenomenon) gain protection from predators, mostly birds, that mistake them for their models and leave them alone. It must work very well, as there are so many kinds of flies that mimic wasps. There are also grasshoppers, beetlees, moths, and other insects that do the same, especially in the tropics, where there are so many more species of insects and so many more birds that eat them.

A study done in Illinois that involved extensive collecting of model Hymenoptera and mimic Diptera showed that the mimics are common in the spring, when adult birds are present as predators, but virtually absent during the period in midsummer when young birds are fledging. Some of them appear again in fall. The authors speculated that the mimic flight seasons were adjusted to miss the time when young, naïve birds were everywhere, birds that wouldn’t know enough not to catch them!

Dennis Paulson 

KINGS OF THE INSECT JUNGLE

Many insects are predators on other insects. Dragonflies and damselflies (order Odonata) come to mind immediately, as all of them eat smaller insects and spiders. But put them up against robber flies (order Diptera, family Asilidae), and they have not only met their match but been bested with ease.

Robber flies eat dragonflies and damselflies regularly, but there are almost no records of odonates turning the tables. One type of predator is clearly superior to the other. I have seen robber flies take insects from most orders, including their own. Size is no limit, as an inch-long robber fly can latch onto a flying dragonfly three times its size and bring it down to the ground instantly with a paralyzing bite. Presumably if the fly was captured, it could do the same thing to its captor.

The two wings of a robber fly are narrow but strong, and they propel their owner through the air with an audible—sometimes impressively loud—buzz. Their flights are usually short, and when you hear that buzz you can often find its source resting on a branch, rock or the ground. They usually perch right out in the open, again like a dragonfly, where they can see potential prey. They have relatively large, forward-pointing eyes as befits a predator.

The thick, tubular proboscis injects venom that is both proteolytic and neurotoxic. The neurotoxin paralyzes the prey almost immediately, and the proteolytic enzymes digest the innards into a liquid soup that the fly sucks out. The proboscis is strong and sharp enough to penetrate the hard cuticle of a beetle.

Many insects are poisonous and distasteful and brightly colored to advertise their unpalatibility. This adaptation must be against birds, because robber flies freely feed on such insects, as do dragonflies.

Robber flies are very bristly. The legs have long, sharp spines to hold onto the prey, much as in dragonflies. The face has a dense coat of bristles, called the mystax, presumably to protect it from the legs and mandibles of struggling prey (but it’s a sure thing that they don’t struggle for long).

Many robber flies are sleek and pointed at the rear, the jet fighters of the insect world. Others are fat and fuzzy, very effective mimics of bumble bees but just as effective as predators. They have been called aggressive mimics, mimicking their favored prey species to get close enough to make a kill.

Fly larvae are legless and look like maggots, and robber flies are no exception. Slim and pointed at both ends, at least some of them feed on the larvae of other insects, usually in rotting organic material such as logs and dead trees or in the soil. Surprisingly little is known about the larval life of this group, however.

With over 7000 species in the world, robber flies are diverse on all the continents. They are relatively uncommon in the wet western lowlands of the Pacific Northwest, just as many groups of insects are less common in our cool, cloudy summer climate. Head across the Cascades to see a lot more of them in the dry, open areas that they prefer.

Robber flies are easily observable, as they are fairly tame, but capturing one in an insect net and looking at it closely allows you to appreciate its adaptations even more. Be cautious, however, as their bite can be painful. I know enough about their adaptations that I have never allowed one to bite me!

Dennis Paulson