A cunning crab spider

Sean McCann recently returned from an epic journey through the rainforest of Guyana in search of caracaras, and is busy blogging about his adventures over at Ibycter.com (so you should check it out!). To make up for not taking me along, he found and photographed lots of awesome spiders for me to blog about. Here’s the first one!


Here we have a male crab spider (family Thomisidae) with its prey: an ant in the genus Dolichoderus*. At first glance, this may not seem particularly exciting. A small, hairy, black spider eats an insect. What’s so special about this scene?

Well, the spider appears to be Strophius nigricans, reported to be a specialist predator of ants. Most animals are not big on eating ants because they are generally distateful and well-defended by strong mandibles, stings, and defensive compounds. So specializing on ants is not particularly common, and tends to come along with some neat adaptations.

Strophius nigricans is not well studied, but I managed to find one paper about its predation behaviour. Oliveira and Sazima (1985) observed a male S. nigricans carrying an dead worker ant (Camponotus crassus) in the field in Brazil, and took it back to the laboratory to make some observations. The spider never let go of his ant carcass – this would come in handy later. In captivity, he was provided with some more C. crassus workers, and here his secrets were revealed.

The Strophius male used his ant corpse as a shield for protection against ants during predation. To eat ants, one must spend time near ants, but they don’t take kindly to intruders. The dead-ant-shield provides a clever disguise. Most ants don’t have especially good vision, and instead rely mainly on their senses of touch and smell. Whenever the Strophius male was approached by an ant, he would present his previous meal, which (obviously) feels just like another ant, and, if recently deceased, probably smells right too. As the spider pursued his prey, always attempting to sneak up from behind, he held the ant corpse aloft. Once within striking distance of an unsuspecting ant, he quickly dropped the shield and bit his new victim. Once dead, this new ant was used as a shield.


Unlike a related crab spider species that preys on ants, Aphantochilus rogersi (a remarkable ant-mimic), Strophius doesn’t look all that much like its victims.  A. rogersi specializes on Cephalotes ants, which have relatively good vision, but its excellent mimicry is probably more important as a defense against visually hunting predators that avoid eating ants. However, a similar mode of defense is not necessarily out of the question for Strophius. The ants it preys on are black and have white hairs on the abdomen. From above, the spider carrying its ant-shield might look just enough like an ant carrying its comrade to fool a potential predator such as a bird.

As it turns out, the conclusion that Strophius nigricans are specialists on the ant species Camponotus crassus appears to be based on Oliveira and Sazima’s observations of the single male discussed above. Here we’ve seen that the spider also takes another species, and it seems that its prey capture technique should work just fine as long as the shield matches the subsequent target prey species. The visual mimicry would presumably be just as effective for any similar sized black ants. This certainly seems like a cool system that could use more investigation!


*Thanks Alex Wild for the ID!


Oliveira, P. S., & Sazima, I. (1984). The adaptive bases of ant‐mimicry in a neotropical aphantochilid spider (Araneae: Aphantochilidae). Biological Journal of the Linnean Society, 22(2), 145-155. (pdf)

Oliveira, P. S., & Sazima, I. (1985). Ant-hunting behaviour in spiders with emphasis on Strophius nigricans (Thomisidae). Bulletin of the British Arachnological Society.

Ghost spiders (Anyphaenidae)

Recently, Sean found a lovely pale spider in his mother’s garden in Victoria. Neither of us recognized it, and after a bit of research, I was able to identify it as a member of the family Anyphaenidae. Spiders in this family are called ghost spiders, and this is the first one I have ever encountered. She is now living in a petri dish on our bookshelf, and has made herself a cozy silk retreat inside a curled leaf.


Anyphaena aperta female (missing her left front leg) photo: Sean McCann

Not much is known about the natural history of these secretive spiders. Apparently they are generally nocturnal hunters that spend their days in their silken retreats. Apart from building these “sleep sacs” under leaves or rocks (and of course spinning egg sacs), ghost spiders don’t have much use for silk.


Anyphaena accentuata in her silken retreat under a leaf. Photo: Ferran Turmo Gort (licensed under CC BY-NC-SA 2.0)

Ghost spider body forms and lifestyles are similar to those of the sac spiders (family Clubionidae), and in fact they used to be considered clubionids. They also have some similarities to ground spiders in the family Gnaphosidae. Members of all three families are wanderers with prominent spinnerets. Luckily for me and others without regular access to a microscope, there are some fairly obvious characteristics that help distinguish Anyphaenidae from other spider families.

Less reliable, but easier to see, is the eye arrangement.

Ground spiders in the family Gnaphosidae often have some oval eyes, whereas anyphaenids’ eyes are all round.


A ground spider in the genus Zelotes (Gnaphsosidae) with her lovely pink egg sac. Note the barrel-shaped spinnerets. I know you can’t really see her eyes – click the link above for a drawing. Photo: Sean McCann

And unlike many sac spiders in the family Clubionidae, the anterior median eyes (AME; the central pair closest to the front of the head) are usually smaller than the other eyes.


Sac spider with relatively large anterior median eyes. Photo: Sean McCann


Anyphaena eye arrangement. The anterior median eyes (central pair in the front row) are smallest. Photo: Dann Thombs (licensed under  CC BY-ND-NC 1.0)


Dorsal view of eye arrangement of an Anyphaena male. Photo: Kyron Basu (licensed under CC BY-ND-NC 1.0)

The real giveaway characteristic of anyphaenids, however, is the tracheal spiracle (one of the openings of the respiratory system in spiders). To get a good look at this, it’s useful to have a Spi-Pot. In other similar-looking spiders the tracheal opening is located close to the spinnerets. In ghost spiders, it is closer to the centre of the abdomen.


Ventral view of a male Anyphaena aperta. Photo: Ken Schnieder (licensed under CC BY-ND-NC 1.0)

The horizontal groove on the underside of this spider’s abdomen (across the darker patch in the middle) is the tracheal spiracle.

Although not much to look at from the outside, the tracheal system of the anyphaenids may provide us with some clues about their natural history. Internally, the tracheae of ghost spiders are much wider than those of sac spiders. This may be because they have a more active lifestyle than sac spiders; they are certainly very fast runners. Males have even larger tracheae than females, which Platnick (1974) concluded is likely related to their extremely vigorous courtship displays. During courtship,  Anyphaena accentuata males vibrate their abdomens up and down so quickly that it becomes a blur. In contrast, sac spider courtship has been described as “sluggish”.


The ghost spiders could certainly use more study, and I’ll be keeping an eye out for them from now on!


Anyphaena aperta again. Photo: Sean McCann



Dondale, C. D., & Redner, J. H. (1982). The insects and arachnids of Canada. Part 9. The sac spiders of Canada and Alaska. Araneae: Clubionidae and Anyphaenidae (No. 1724). link

Platnick, N. (1974). The spider family Anyphaenidae in America north of Mexico. Bull Mus Comp Zool Harvard Univ. 146: 205-266. link