WormLearn Arachnids

Arachnids Map

Arachnids

Class Arachnida is a division with the Phylum Arthropoda, or "joint-footed animals". This is a diverse group of organisms which also includes (but is not limited to) the Insects, the Myriapods (centipedes and millipedes), the Crustaceans and the Pentastomatids, or tongue worms. Arachnids play an important role as agents or carriers of disease, as well as acting as ectoparasites. For the purposes of this resource, the arachnids have been classified as Mites or Ticks. Both are members of the order Acarina. Other arthropods of medical importance, such as those which cause envenomation (eg.the spiders and scorpions) are not covered in Wormlearn.

Mites

The true parasitic arachnids are the mites. These are tiny, eight-legged arthropods that burrow into the skin of their host. The best known mite which infects humans is Sarcoptes scabei (sar-COP-tees SCAY-bee-eye), the human scabies mite. This mites are so small as to be all but invisible to the naked eye. Under the microscope they resemble soccer balls with short stubby legs - long legs would get in the way while burrowing through the skin. Scabies mites live at the bottom of deep burrows they dig into the skin, feeding on the skin cells as they go. They emerge at night time to mate and to dig new burrows. Because they are well protected in their burrows, treatment is troublesome, and must be repeated several times. The burrowing activities of the mites causes an almost irresistible itching. The infestation is highly contagious (transmitted by touch) and spread rapidly where people are kept in close quarters to each other. A relative of Sarcoptes scabei causes one sort of mange in dogs.

A less well known parasitic mite is Demodex folliculorum (DEM-oh-dex foll-ICK-you-LOR-um). Demodex has an elongated body which allows it to live in the hair follicles (particularly the eye-lashes) of the host. Normally, these mites feed on dead skin and other debris, although sometimes their activities can allow bacteria into the follicle and infection can occur (eg. a sty). One species of Demodex causes a form of mange in dogs.

Images of Sarcoptes scabei and Demodex folliculorum, as well as the bird mite Dermanyssus gallinae (DER-man-ISS-uss GAL-in-ee), are contained in Wormlearn

Ticks

Ticks cannot truly be called parasites because, even though they may staying feeding for a number of days, they leave the host as soon as they have stopped feeding, living for periods of time off the blood they gained from their last meal. Ticks are arachnids with dorso-ventrally flattened bodies. When feeding, their bodies may swell to many times their original size.

Ticks are divided roughly into two groups : hard (or Ixodid) ticks and soft (or Argasid) ticks. Soft ticks, as the name suggests, have large soft bodies, They normally feed of animals with thick fluffy coatings (eg. birds) where they are protected by the body coverings of the host. When looking at specimens of these, the head and mouthparts appear to protrude from beneath the body of the tick. Hard ticks are usually found on animals with less body coverings and have a tough "shield" or scutum to protect them. The head on these ticks appears to protrude from the anterior end of the organism. However, finding the scutum or even the head and mouthparts may be difficult in hard ticks engorged with blood.

Ticks may be carriers of disease (eg. Scrub Typhus, Lyme disease, Babesia) or they may cause dangers by themselves. The scrub tick Ixodes holocylus (ick-ZO-dees HOL-oh-SY-luss) produces a paralysing poison in its salivary glands. In small animals and young children, the amount of poison present might be sufficient to kill.

Images of the soft tick Argas persicus (AR-gass PER-sick-us), and the hard ticks Amblyomma cajenense (AM-blee-OH-mah cay-jen-EN-see), Dermacentor andersoni (DER-mah-SENT-or ANN-der-sun-eye) and Rhipicephalus sanguineus (RYE-pee-KEFF-ah-luss san-GWIN-ee-uss) are contained in Wormlearn