Here’s a tiny, colorful crab spider perched on the milkweed flowers on our balcony. She’s similar to some we’ve found in the past, and is likely one of our 16 local species of Mecaphasa. But she’s more dramatically patterned than others we’ve found. One indicator that a crab spider is in the genus Mecaphasa is how spiny it is, how many setae, or sensory hairs, it has on its abdomen. Some crab spiders are very smooth, with almost no setae. But Mecaphasa has many setae, giving it a hairy or spiny appearance.
It’s also helpful to look at the spider’s eyes to distinguish its species, though even in these macro photographs it’s hard to see the eyes in much detail. Mecaphasa have two eyes, the anterior lateral eyes, that are noticeably larger than the other eyes. Some other genera of crab spiders have all eight eyes about the same size.
Like other crab spiders, this one has very long first and second legs, which are used to catch prey — bees or flies that land on the flowers — while the third and fourth legs are rather stubby and are kept bent up under the body for leaping towards prey.
Our spider shares the milkweed with several other critters, including a couple of Tenodera sinensis mantids and some milkweed bugs, Oncopeltus fasciatus, which lay their eggs on the plant so their nymphs can feed on the seeds later in the year.
Steven E. F. Brown 