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.

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