We can often see, in our vermicompost, some very small organisms which look like small spiders, with only a few millimeters in length, moving rapidly through the top or surface of the remains, or who are immobile and grouped. They are mites.
Dust mites are tiny living beings who belong to the arachnids group. They are, therefore, relatives of spiders and scorpions. Mites can be found in almost all the environments, both terrestrial and aquatic, including marine. It is normal if we see them in our vermicomposter. In fact, they are one of the most abundant animal groups in different soil layers, and we must remember that the process that takes place in the vermicomposter is the same as with the organic matter in nature. It has been discovered that there are around 45000 species of mites, but this is estimated to reach about a million! Due to this abundance, and to the fact that they have colonized different environments their anatomy can be very variable. This article will give some hints of basic anatomical features.
Like all arachnids, they have no wings or antennae, and they have four pairs of legs. A characteristic of mites is that they can be noted that the body does not have clearly differentiated into two regions. In its own internal morphology a prosoma and a opisthosoma, which are the two segments of the body spiders, can be differentiate, but this segmentation in almost any species of mite is reflected on the outside. We can distinguish the gnathosoma, which is the mouthparts of the mite, and idiosoma, what is the body.
Gnathosoma is found in the mouth and in some appendices: the chelicerae and pedipalps. The chelicerae are feature food appendices, ending in forceps. This clip is usually comprised of a fixed finger and a movable finger. The moving finger is sometimes modified. Some mites are plant parasites that make the finger become a stylus that allows them to pierce the cells of plants and feeding from them extracting its content. Others have a modified form of harpoon, so that when they bite a host they get stuck. That is the case of ticks. The pedipalps have mainly a sensory function. For this purpose they have a number of sensory hairs. In the oral region they are also silk-secreting glands, which produce silk.
Idiosoma is found in the legs. Almost all species have eight legs in adulthood, but very often some larvae have only six. Both idiosoma and legs also have sensory setae, as well as other receivers. For example, in the front of the ocelli idiosoma we can find a simple light receptors. We also find the genital, anal and breathing holes. There are two different respiratory systems, they can breathe through the skin, and also through holes, known as stigmata, which connect to an internal system of tracheae, where it takes place the exchange of gases. Many groups of mites have the two systems, but there are groups that only have one of them. In reference to reproduction, the mites have separated sexes, ie males and females can be found. The exchange of semen is carried out by silk structures, the spermatophore, the male deposites the substrate or he gives it directly to the female. In general, the life cycle includes different larval stages. As stated previously, in most species the larva has six legs.
They are adapted according to the environmental conditions where they live and the food they feed. According to food we can find the following groups:
- Predatory mites: free-living mites that feed on insects, nematodes (small worms) of Collembola and other soil organisms. The animals move fast, with harder skin depending on feeding species, and chelicerae adapted to the capture prey. Some water mites, for example, transforme their pedipalp into a clip for this purpose.
- Mite parasites: they eat other living organisms, both plant and animal. There are species which are very harmful to crops, and others that are harmful for animals, including humans. It is worth mentioning the case of ticks, which belong to this group, and who feed on mammalian blood. Some species are used to control insect pests and weeds in the fields because they feed from them to death.
- Saprophytic mites: their diet is based on traces of organic matter. Their skin is softer and they have slower movements than the predatory mites. We can divide the saprophytic mites in microphytofags and macrophytofags. The first feed on fungi and bacteria, while the last ones feed from dead plants, including wood. These are the primary decomposers, because they are very greedy, and can digest about 20% of their body weight daily. Moreover, they leave prepared organic debris so fungi and bacteria can act on them, since the crushed and chemically degraded. Microfitófagos mites, in turn, feed on the fungi and bacteria, contributing to its spread and to stimulate their growth.
As it has been explained, the mites are directly involved in the cycle of organic debris, mostly plant in soil. That is the reason why we should not worry if we see them in our vermicomposter. These mites are not what some of us are allergic, they are not bad for us, quite the contrary, our composter will become a real ecosystem!