Figure 1. Beetle (family: Passalidae) with three different species of mites (indicated by A, B, C) attached. (See Ermilov & Frolov 2019b.)
Mites may be found worldwide in almost any habitat imagined, from arctic tundra to hot deserts, from marine habitats to forests. Collectively, they also eat an extraordinarily wide variety of food. However, when the habitat becomes too crowded, or the specific food source has run out, they have to disperse to the next optimal space. Sometimes it is as easy as walking to the next spot, but sometimes the next food source is quite a distance away, and being so small and wingless, walking is just impractical.
To travel between habitats, mites can firstly disperse passively, e.g. with wind or water currents. However, this way is ideal only when there are many individuals and the sought- after habitat is abundant. If some of them miss the mark and die, the loss for the species is acceptable. The second, more reliable, method is to catch a ride on something bigger that is going in the same direction.
Phoresy is when one animal (the phoretic) actively seeks out and attaches to the outer surface of another animal (the carrier) for a limited period of time, solely for transport. In easy terms, phoresy is the act of ‘catching a ride’. This association can be mutually beneficial for carrier and phorectic, or not. In the case of mites, there are several records of a variety of animal ‘taxis’. The most common carriers are certain types of beetles, bees, ants, flies, termites, and sometimes cockroaches, fleas and spiders. Even vertebrates such as birds, rodents and frogs had been recorded to transport mites, but only in rare cases. Of course, picking the carrier is not random. Animals living in close contact with or in the same habitat as the mites are the ideal carriers. For example, mites living in flowers can easily catch a ride with bees eating the nectar of the flower, because they are in close contact, occurring in the same habitat; catching a ride with the bees will take them exactly where they want to go. In order to travel on animals, mites are often adapted for the ride.
Most phoretic mites use their claws to hold on. The claws can be normal, or modified for a tighter grip. Modified claws on the first and second legs have been reported on an oribatid mite species (Ramusella bochkovi) attaching to earth-boring dung beetles. Sometimes up to a 100 mites were observed on one beetle, sharing the same habitat as the mite. In another instance, oribatid species were found on passalid beetles (Figure 1), but in this case the mite species had no obvious adaptations for attaching to the carrier. It was speculated that normally, the gripping power of oribatid mites is extraordinary strong, and secondly that there are many unexposed places on the carrier where the mites can hide while travelling. A scutacarid mite species (Scutacarus acarorum), phorectic on bumblebees and also hyperphoretic on nymphs of a gamasid mite (Figure 2) on the bees, has very large claws on the first and second pair of legs with which they grasp the carrier’s hairs (Figure 3).
Some mites, for example of the family Uropodidae, may attach to the carrier by means of an anal pedicel (Figure 4), which is a liquid strand produced from a gland at the anal opening, which hardens on contact with air. Mites can also attach to the carrier with their chelicerae (‘teeth’), palpal hooks (mouthparts), or with sucker-like structures.
Some mites may secrete themselves in a cavity, called an acarinarium, a structure common in female bees and wasps (Figure 5). The term literally means a place where mites are kept (Latin: arium: a place where things are kept; Acari – mites). The adaptation by the bees suggests that some benefit is derived from carrying the mites, and indeed there is. The mites eat fungus growing in the bee nests and also repel other mites or parasites that may be detrimental to the bee.
In another interesting case, a scutacarid mite (Imparipes sp.) was observed to travel on termites from place to place within the same termite nest. They were not observed on the winged termites, and were therefore not on their way to a different area such as a new nest. The study actually called these mites ‘lazy hitchhikers’, because actually they do not need a ride. This form of phoresy might simply indicate an energy-saving method of travelling. These mites have no claws on leg one, but possess large pad-like empodia (lobes) (Figure 6) on the ends of the first and second legs, with which they probably attach to the termite. They attach between the coxae of the legs of the termite, where they are protected. This area also has less hair and more smooth areas for attachment with these adhesive pads (empodia).
So, mites have adapted well for travel – by attaching to other animals they can travel by land, air and sea.
Baumann, J., Ferragut, F. & Šimić, S. 2018. Lazy hitchhikers? Preliminary evidence for within-habitat phoresy in pygmephoroid mites (Acari, Scutacaridae). Soil Organisms 90(3): 95-99.
Ermilov, S.G. & Frolov, A.V. 2019a. Ramusella (Dosangoppia) bochkovi (Acari, Oribatida, Oppiidae), a new subgenus and species of oribatid mites phoretic on Ceratophyus polyceros (Pallas, 1771) (Coleoptera, Geotrupidae) from Russia. Systematic and Applied Acarology 24 (2): 209–221.
Ermilov, S.G. & Frolov, A.V. 2019b. New and interesting oribatid mites (Acari, Oribatida) phoretic on Aceraius grandis (Coleoptera, Passalidae) from Vietnam. Systematic and Applied Acarology 24(5): 945–961.
Krantz, G.W. 2009. Habits and Habitats. In: Kranz G.W. & Walter D.E. (eds). A Manual of Acarology, Lubbock (TX), Texas Tech University Press: pp 64-82.
For another example of phoresy see: Neethling J.A. 2019. Pseudoscorpions: Arachnid hitchhikers. Culna 73: 22-23