Chelicerae = first pair of appendages of arachnids, mouthparts used for grasping of prey, sucking of food, or stinging; in the case of spiders they contain a venomous gland (see for example the chelicerae of Acaru siro)

Pedipalps = second pair of appendages of arachnids, modified for various (manipulation, sensory, reproduction, predation) functions, claw of the scorpions

Compound eyes = eye of many insects (see the big eyes of the fly) = they consist of hundreds of optical receptors, which can be sensitive to light, colour and movement. Insects can see an image of their environment that is reconstituted from the different information transmitted by each optical receptor. Some insects even have ultraviolet vision! 5 compound eyes

about mites > the class Arachnida
Mites belong to the class Arachnida

What are arachnids? Mites, scorpions, spiders and ticks belong to the Arachnida. They all have a body that is, in general, segmented into two parts: the cephalothorax (the head attached to the chest (thorax)) and the abdomen. Generally, six pairs of appendages are attached to the cephalothorax: a pair of chelicerae, a pair of pedipalps and four pairs of legs. Arachnids don’t have antennae, wings and they cannot fly. They have simple eyes that are sensitive to light intensity; they do not see images but just hazy spots of lights. These are, together with the four pairs of legs, the main differences between arachnids and insects.

Insects (flies, bees, ants, butterflies...) have only three pairs of legs, compound eyes, one pair of antenna and wings. Insects also have complex compound eyes; they perceive light variations, detect movements and are able to see complex images of their environment.

Among arachnids, mites have a rather particular morphology: in mites, the cephalothorax and abdomen are fused. Therefore, the two different regions of the body are no longer distinguishable (see the photo of Tetranychus). The morphology of mites is extremely variable. Most mites are no longer than one millimetre. One of the smallest mites, Eriophyes ribis, is 0.01 cm long. Ticks can, when they are filled with blood, exceed 2 cm: Amblyomma clypeolatum , an Asian tick, can reach 3 cm in length. Besides size, mites can also differ in colour, leg-structure, mouthparts, body-hair…

To date about 50 000 mites have been described. However, scientists believe this represents only a small fraction, about 5%, of the total number of mite species. There is still a lot of work to do!

Mites have, in general, four stages in their life cycle: egg–larvae–nymph–adult. Larvae are very special; they have only three pairs of legs instead of four. The nymph looks like a small adult (with four legs) but cannot reproduce; their reproductive organs are not mature. Depending on the species and on the environmental conditions (T°, humidity...), the time required to complete the life cycle varies enormously: in the mould mite (Tyrophagus), a cycle can be completed in 8 days. At the opposite end of the scale is a mite living in the Antarctic (Alskozetes antarcticus), whose life cycle typically lasts more than 5 years from egg to egg.

  • Mites on an insect (photo: L. De Vos)
  • Chelicerae of Acarus siro (L. De Vos)
  • Flies are insects. This is a fly from the family of the Tachinidae
    (Insecta, photo: C. Salin)
  • Compound eyes of a domestic fly,Musca domestica (photo: L. De Vos)