Swift Crab Spider

Like many members of the Thomisidae family, M. celer have the characteristic long front two pairs of legs and short back two pairs. Adults show sexual dimorphism not only in size but in color, and mature males are often darker than females and have distinctive red bands on their legs. Males also tend to have whiter abdomens with four or more pairs of brown spots that form a V-shape, starting from the mid-abdomen and terminating at end of the abdomen. The male body is also covered in noticeable dark hairs (setae). There can be difficulty in identifying M. celer from a male alone; males of M. celer appear quite similar to males of Mecaphesa dubia and Mecaphesa rothi, and due to natural variation in physical traits that can occur within a single species, distinguishing between these three species can be quite difficult. Female M. celer, meanwhile, are more distinct from other species. They tend to be yellow in color and have large, round abdomens much larger than their heads, unlike the male, whose abdomen and head are roughly equal in size. Females can actually vary in their coloration, with one variant being a bright, solid yellow with barely any brown spotting, another variant being yellow with reddish-brown streaking on the abdomen, and a third variant being completely white with dull brown streaking on the abdomen.

As an ambushing spider which often preys upon pollinating insects, M. celer usually lives in close association with flowers and upper plant parts. The spider is often seen on shrubs, bushes, and small trees, as well as on crop plants, such as sorghum, cotton, and alfalfa. One study of an Eastern Texas cotton field found that M. celer comprised 5.3% of all spiders collected in the field, indicating its abundance in agricultural fields.

Because M. celer feeds on insects visiting the flowering parts of plants, it can act as a valuable biological agent of insect control in crops. Members of the Thomisidae, as well as Salticidae, Argiopidae, and Oxiopidae spider families, have been shown to feed mainly in the upper part or head of the sorghum crop, where the grain is produced. Because sorghum is grown for the grain it produces, M. celer and other Thomisids provide important protection for farmers against the destruction of the crop's value.