spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...

s, the calamistrum is a row of specialized leg bristles used to comb out fine bands of

silk Silk is a natural fiber, natural protein fiber, some forms of which can be weaving, woven into textiles. The protein fiber of silk is composed mainly of fibroin and is most commonly produced by certain insect larvae to form cocoon (silk), c ...

. It is only found on cribellate spiders, that is, spiders that possess the spinning organ known as the

cribellum Cribellum literally means "little sieve", and in biology the term generally applies to anatomical structures in the form of tiny perforated plates. In certain groups of diatoms it refers to microscopically punctured regions of the frustule, or ...

. The calamistrum and cribellum are used to form the hackled bands of silk which are characteristic of the webs of these spiders. The calamistrum is found on the upper margin of the

metatarsus The metatarsal bones or metatarsus (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges ( toes). Lacking individual names, the metatarsal bones are ...

of the hind legs. Each bristle of the calamistrum is serrated on one side and smooth on the other. The length of a spider's calamistrum is always equal to or greater than the width of the cribellum. The ratio between calamistrum length and cribellum width varies greatly, however, even among related species. This is likely due to differences in spinning behavior and as well as differences in the size and shape of the legs and abdomen. When male cribellate spiders reach

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans, it is related to both puberty and adulthood. ''Puberty'' is the biological process of sexual maturation, while ''adulthood'', the condition of being socially recognized ...

, they either lose the cribellum and calamistrum or retain them in a vestigial form. Comstock-calamistrum

References

{{Spider nav Silk Spider anatomy