Crab spiders

voracious little predators

Crab spiders typically are sit-and-wait hunters that ambush insects passing close by, grabbing a victim with two sets of long claws. Small prey can be captured with just the claws, but larger prey is subdued by injection of potent venom that quickly immobilizes the prey. Most of these injection bites are placed between head and thorax or between thorax and abdomen of the victim. Placing its mouthparts in a wound made by the claws, the spider sucks out a little body fluid, mixes it with its own stomach fluid, and reinjects the mix. The mixed fluids go back and forth, with more and more stomach fluids going into the prey. Those fluids turn the soft internal tissues of the prey into liquid, which is then sucked up by the spider. In many cases, feeding begins with the head of the prey, and if the spider is already well fed, only the head contents are consumed; the spider then discards the carcass. Such selective food consumption leaves the question “Why?”. Perhaps the eyes and brain of the prey insect offer special, critical nutrients, or maybe just the most calories. (We see something similar when bears eat just the brain or the eggs from a captured salmon.)

Although some crab spiders hunt in the leaf litter or on tree trunks, the kind we have here (Misumena vatia) habitually hunts on flowers, waiting to nab the visiting insects. Dandelions sometimes make a nice hunting platform, even though they close at night or in the rain. Sometimes the spiders lurk in clusters of flowers, such as the inflorescences of lupines. Misumena vatia can change color from yellow to white and back again, depending on the background color of the flower it sits on, although it may take more than two days to effect the change. So if you see a yellow crab spider on a white thimbleberry flower, you know it hasn’t been there very long.

Finding a patch of flowers to use as a hunting base is probably mostly a matter of luck. A newly hatched spiderling can ‘balloon’ on a breeze that catches its silk thread. If it lands in a good flower patch, that’s a good start. But older, bigger spiders don’t move very far—adult females move only a few meters at most, to find good hunting sites. A variety of flowers within a short distance of each other is required, because the tiny spiderlings have to catch tiny prey, which would probably be most common on small flowers (such as goldenrod), but full-grown spiders need bigger prey and can use bigger flowers. So, to establish themselves successfully, our crab spiders need a patch that provides both small and large flowers, with a flowering season long enough to provide that variety.

In Juneau, where do they find the patches with a long-blooming array of flowers? On some roadsides (until they are mowed) and meadows such as those along Cowee Creek, and maybe in some backyard gardens.

Because adult females are heavy, especially if full of eggs, they can’t move around much, so they have to deal with whatever insects come to the flower they are resting on or another flower very nearby. A detailed study in Maine showed that bumblebees are an important prey, although they are much bigger than the spiders and difficult to capture. In fact, capture success seldom exceeded three percent of attempted attacks. So a lot of bees would have to come by, for the spider to do well. Sizable moths are also good prey, typically available at night, but many flowers close at night and draw no moths. A good foraging patch is one that attracts lots of bees or moths, offering many chances to capture a good prey.

Crab spider capturing a bumblebee. Photo by Bob Armstrong

A hungry spider also attacks smaller prey, such as hover flies or dance flies, but adult females lose weight on a diet of small insects. Furthermore, the females need to be very well fed in order to produce their eggs; they can lay bigger egg clusters if they are very well fed. In the Maine study, few females captured enough food to produce the maximum possible number of eggs. Juveniles, being small, do well on small prey.

What about the males? They have to feed too, and go about it like the females do. But they are much smaller than females (females are more than ten times bigger) and do not depend so heavily on catching big prey. They don’t have the high costs of egg production and they spend their energies chiefly on running about, looking for females to mate with. When a male finds a recently molted, virgin female, he hops onto her abdomen and inserts sperm into her two genital openings using his pedipalps (appendages next to the ‘jaws’ on the head). Males regularly mate more than once, although it takes the better part of a day to recharge their pedipalps. (Females do so much less often). An older female is generally not as receptive and may be aggressive. In any case, the first male to mate with a female is likely to be the father of the brood, so there is less pay-off to the males from such a mating. An aggressive female may sometimes eat an attentive male, but that is not common in this spider: the males are agile enough to escape quite readily and too small to be a rewarding lunch.

Females lay their eggs inside a folded leaf, suspended in a network of silk, and guard the nest by sitting on the outside. Sitting on the outside, rather than guarding inside the nest as many other spiders do, helps keep away parasitoid wasps, a potentially major source of mortality for spider eggs. A female wasp tries to lay a single egg in the spider’s egg mass; one wasp larva can consume all the spider’s eggs, except in the largest egg masses. A defending crab spider often knocks the wasps off her nest. There is usually one brood per season, at least in Maine and probably here as well, but there could be more, farther south. Egg production typically happens in midsummer, and the eggs hatch almost a month later. The tiny juveniles need small prey that come to small flowers, because they need to feed before hibernating in the leaf litter. Juveniles that are well fed and bigger survive the winter better than small ones. The following summer, they grow some more and molt again, and spend the next winter hibernating. The next spring or early summer, they molt to the adult stage, and probably do not live through the next winter. This basic pattern may, however, vary with conditions.

Crab spiders are not common in Juneau, perhaps in part because there aren’t many good patches of habitat. So if you would like to see a crab spider in action, check out the videos on the following website:

Thanks to Bob Armstrong for photo and videos, to Doug Jones for the loan of a book by D. H. Morse that details much about these crab spiders in Maine.

Yellowjackets and paper wasps

On a recent walk on Gustavus’ nagoonberry trail, the larger forms of wildlife were absent or in hiding. But my naturalist friend and I spotted a wasp clinging to one of the last goldenrod inflorescences, not moving at all, just resting. That observation led to a brief discussion of “wasp” versus “hornet” versus “yellowjacket”—what’s the difference? So later that day, we did a little online research.

The term “hornet” is officially applied to certain European wasps, one of which is found as an alien in eastern North America. However, we tend to be quite casual in how we apply common names for organisms, and sometimes we just call all wasps ‘hornets’, even though that is not quite correct.

“Wasp” is a good general term for a variety of Hymenoptera that are clearly related to bees but different enough to fall into several taxonomic families. Back in the Midwest, I sometimes saw the huge, beautiful wasps known as cicada-killers as they searched among the flowers for prey. That one doesn’t occur here, but we do have other kinds of wasps, including two that make nests where we can see them.

Up under house eaves, in wood sheds, under car ports, we sometimes see the nests of paper wasps (genus Polistes). These nests are made of chewed-up wood fibers, i.e., paper. Each one consists of a more or less horizontal cluster of brood cells, suspended on a cord. Brood cells house the growing larvae, fed first by the queen and later by siblings that are workers from the first batch of larvae. The queen retires from feeding her offspring then, and just lays more eggs. Adult paper wasps feed on nectar, but the larvae are fed chewed-up insects such as caterpillars.

Another kind of wasp includes several species called yellowjackets. These wasps also chew up wood fiber to make their paper nests, but there are usually two or more clusters of brood cells, one suspended below another, and the whole works is enclosed in an oval, papery covering. (There is more paper involved with these nests than with those of the so-named paper wasps, making one wonder about the naming process). Yellowjacket nests may be suspended from branches or rafters or be constructed underground.

Years ago, on some long-forgotten project in the Midwest, I stumbled over a subterranean yellowjacket nest (a kind known locally as bald-faced hornets…). This angered the whole colony and they took it out on me. Somehow they knew that I was the guilty disturber and not my nearby research companion.

Yellowjacket nests commonly have more brood cells than do paper wasp nests, so there are usually more workers. Neither kind of wasp stores honey in the cells, unlike bees. The wasps feed their larvae on chewed-up insects, while the adults eat both insects and nectar. Some species feed only on live prey, while others also visit carcasses, picnic tables, and succulent garbage. Certain species usurp the nests of other yellowjacket species and the host workers raise the usurper’s brood—they are brood parasites, the cuckoos of the wasp world.

Yellowjacket queen on willow catkin. Photo by Bob Armstrong

The seasonal cycles of yellowjackets and paper wasps are similar. Toward the end of the summer season, a new queen emerges from her brood cell. Males are also produced at this time, and the queen finds a mate. All the males and workers die before winter, but the new queen and her fertilized eggs hibernate in the soil. She emerges in spring, builds a new nest, and installs that first batch of eggs in their brood cells, starting the cycle again.

A good walk often takes me into unexpected thought directions. It starts with a simple observation (in this case, a wasp on a flower), but one thing leads to another, and it’s fun to see what directions the thoughts take.

End of summer

low water, autumn flowers, mountain fish, and alder eaters

It’s really fall, now—the autumnal equinox has passed, and we’ve all noted the rapidly shortening days. The fireweed leaves are mostly reddish and the seed pods have shed their offspring to the winds. A friend observed that the curly valves of the pods looked like the plants had been given perms—all by the same hairdresser.

Late summer, and the long drought reduced my home pond to little more than a mud puddle. Even so, two broods of mallards visit every day, no doubt drawn by the seed spilled from the feeder that hangs over the erstwhile pond. The young ones are well-feathered and nearly as big as their mothers. The hordes of pine siskins that monopolize the feeder are very messy and lots of seeds fall down where the ducks gobble them up. The two duck families don’t mix at all and typically push each other around.

There weren’t many fish in the pond earlier in the season—just a few juvenile salmon and some sticklebacks. They probably weren’t doing very well in the shallow, warm, and turbid water. I watched a mallard grab a young salmon from the shore and walk off with it before gulping it down. Fish-eating by mallards is not as odd as it might seem: when I worked at the hooligan run in Berners Bay, I often saw ducks eating dead or moribund hooligan.

Late summer, and at low elevations the fireweed is finished blooming. Purple asters are now on show in many places. In some of the meadows near Eagle River there have been nice displays of long-blooming grass-of-Parnassus and—a special treat—some lovely stands of felwort with its small, blue, star-shaped flowers, just starting to bloom. Felwort always seems to bloom late in the summer, when most other flowers are done. It’s something I look forward to.

Felwort. Photo by David Bergeson

All summer long, I’ve had a pot full of the little pansies called johnny jump-ups near my door. One day in July, I came out that door and said “I’ve been robbed!” The two deer that mutilated the fireweed in the front yard, nibbling the upper leaves down to ragged stumps, had come round and mowed down my little johnnies to a height of about three inches. No flowers left. Well, I nursed those plants back to something like their sprightly selves and they again flowered briskly. But the deer came back and this time they left only one-inch stubs. After some considerable time for recuperation, now the johnnies are trying again, but there are many few flowers this time.

One August day I watched a young buck demolishing more fireweed in front of the house. He slowly wandered along the edge of the drying pond where I have some poorly tended terraces. As I watched, he started chomping on the Canterbury bells. That was just too much. I eased my way slowly toward him and when he finally noticed me (he being much too busy eating!), he went the other way, with determined sedateness and his dignity intact, and so disappeared into the woods. But I reckon the deer are not done with my flowers!

Up at Cropley Lake the fish were rising. These are resident Dolly Varden that mature at a small size, much smaller than the sea-run dollies. There are also resident dollies in the creek that flows from the lake, but I’ve been told that the population in the lake is probably quite separate from the one in the creek, with little or no genetic mixing. The lake population is thought to have been there a long time.

In mid-August, on North Douglas, I happened to notice an alder shrub whose leaves had been reduced to skeletons. Some critter had eaten the blades and left just the veins. A closer look found some of the perpetrators—a cluster of fuzzy white caterpillars. These turned out to be woolly alder sawfly larvae. Later, driving out the road, I noticed other alder stands that were nearly leafless.

In consultation with the helpful FSL entomologist, Liz Graham, I learned that there are at least three kinds of sawflies working on alder leaves. The striped alder sawfly is a native species. The woolly alder sawfly and the green alder sawfly are not native here, although the woolly one seems to be naturalized and the green one has been in this area for several years. Heavy sawfly infestations are patchily distributed, nothing like the widespread swaths of browned hemlocks, whose previous-years’ needles are being decimated by the hemlock sawfly this summer.

Sawflies are not true flies; they are related to bees and wasps. They get the first part of their common name from the long, serrated tube through which females deposit their eggs. I’ve read that the three species deposit eggs in somewhat different parts of the leaf: woolly ones on the underside of the leaf and in the midrib, green ones on the upper leaf surface, and striped ones along the leaf petiole. The eggs take one or two weeks to hatch. The green alder sawfly burrows into bark and wood when it is ready to pupate, but the other two species pupate in the soil; the adults emerge the following year. It takes more than one year of defoliation to kill an alder, but defoliation by the insects means that there is less nitrogen from decaying leaves put into the soil. The nitrogen-fixing bacteria that live in alder roots put lots of nitrogen into the plant, and this gets recycled back to the soil when the leaves decay. I wonder about possible ecological consequences of breaking that nitrogen-recycling pattern.