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.

Good finds in Gustavus

stealthy spiders, ambitious amphibians, strange ferns, and more

A summertime walk through woods and meadows is almost always good—birds are singing, flowers are blooming, and there’s always nice fresh air. But sometimes all the little pleasures form a base on which rest some observations of particular interest. Here are a few good ones from a recent trip to Gustavus.

–Dandelions had mostly gone to seed, so fields that had been golden with flowers were now white with plumes on mature seeds ready to disperse on the wind. But here and there we found a laggard flower, still yellow and conspicuous on the background of white. On one of these late bloomers there was a bumblebee, a strangely immobile bee. Looking more closely, we saw a yellow crab spider with the bee in its clutches. Crab spiders are venomous (to insects), immobilizing their prey and then sucking out the juices. Dinner was in progress and the bee would fly no more. Crab spiders are generally ambush-predators; some of them lurk on flowers in hopes that a tasty insect will alight. The color of the spider often matches the color of the flower on which it awaits a victim.

photo by Kerry Howard

–The ponds at the gravel pits are a great place to see shorebirds, swallows, and kingfishers. There were sticklebacks swimming around and, in June, there were gravid females full of eggs. One pond held many thousands of toad tadpoles, swarming in the shallows where the water temperature was salubrious. They came in a variety of sizes—some at least six times bigger than the smallest. A female toad can lay thousands of eggs; the hordes of tadpoles that we saw undoubtedly had many mothers, which probably laid their eggs at somewhat different times, accounting for the size variation. None of them had begun to transform into toadlets; no little legs were visible. A dense pack of tadpoles clustered around a silvery object, each one trying to grab a mouthful. Looking closely, we discovered that the silvery object was a dead stickleback. Toad tadpoles commonly feed on algae and detritus, but they are also known to scavenge carrion and even the dead bodies of their comrades. Toad (and frog) populations have declined dramatically almost everywhere, and it was heartening to see this large aggregation of juveniles.

–Gustavus is noted for (among other things) its wide sandy beaches. On our way out to one of them, we heard some odd sounds, rather like the hooting of a small owl. As we listened carefully, however, it became apparent that several snipes were performing their aerial territorial display. It’s called ‘winnowing’, and it’s made by the rapid passage of air over the spread-out tail feathers, usually as the bird dives toward the ground from high in the air. Usually the male does this but sometimes females do too. I hadn’t heard this display for a long time and it was a gladsome sound.

–Out on the sandy beaches we found windrows of long, flexible tubes that were the former housing of certain marine worms. The worms were long gone, possibly starved at the end of winter when food is scarce. Then the tides presumably stripped the empty tubes from their attachment points and piled them up on the beach. This observation stimulated a lot of conjecture but no concrete answers.

–On the vegetated sand dunes there were lots of the strange little ferns called moonworts (a.k.a. grape ferns). They don’t look at all like ferns to the layman’s eyes, because the fronds are generally not very lacy or branched. We found many that appeared to be the common moonwort, but there were also a few much more robust individuals that were certainly a different species. On one of the postglacial-uplift meadows we found another kind, one that is now classified in a different genus; the fronds on this one (so-called rattlesnake fern) are somewhat more ‘fern-y’. All three of these are widespread species in North America and even beyond, but they are so odd that it is always fun to find them.

–The pilings of the public dock usually offer something even to a casual observer. Enormous white anemones, far larger than any we usually see in the rocky intertidal zone, wave their tentacles if the tide is in. Sea stars cling to the vertical surfaces too, but the largest ones have trouble hanging on when the tide goes out and leaves them above the waterline. Colorful sponges and tunicates add to the array. Sometimes there’s a giant whelk laying a coil of egg cases. Small fish sometimes gather under the docks and are visible between the pilings. And while one inspects the fauna on the dock, barn swallows are swooping overhead, gathering flies and mosquitoes for their chicks.

–We searched for lady’s slipper orchids (of which more, later). One clump of flowering stems still included a stalk with last year’s seed capsule, well dried. Someone opened the capsule to see if any seeds were left and found, instead, a tiny spider guarding her minute ball of orange eggs. We were sorry to have destroyed her safe-house!

–Sweetgrass grows in many Gustavian meadows and some of us stopped to braid some stems. Braided sweetgrass is used, especially by Native Americans, to construct baskets and decorative items, and we had to try just a simple braid. As we concentrated on our task, we heard thundering hoofbeats, getting rapidly closer. Turning around, we saw a fast-trotting female moose, followed by a young calf. They were so intent on getting away from whatever startled them that they ignored us and passed by, barely thirty feet away, and off they went, full tilt.