How Autotomy and Regeneration Occur in Arthropods

The concept of autotomy is used to describe a defensive mechanism employed by animals to evade predators by way of distracting them or escaping from being captured. The power of regeneration, though not so extensive as is found in annelids, is still pronounced in crayfish. It is however, limited to the regrowth of appendages or portions of them which are broken off or damaged. Sometimes a limb, when caught or held under a stone, or even when stimulated, will be broken off by reflex action of the animal itself. The process is known as autotomy. It has been thoroughly studied in the lobster and probably applies also to the pereiopods of Astacus. The ischipodite of the limb bears a groove near its base. From a point just proximal to the groove two autotomizer muscles are inserted. When they contract, the limb is bent over against a process on the coxopodite and snaps. The breakage plane not damage any muscle, as it lies between the origins of one set and the insertions of the next.

Immediately proximal to the break is a connective tissues membrane stretched across the limb. It has a small perforation in the centre through which the artery and nerve passed down the limb. At autotomy they are withdrawn through this hole into the body. The blood forms a clot over it by cytolysis of special leucocytes and soon the epidermis spreads over the inside. A delicate layer of chitin is formed to protect it. At each moult the stump enlarges until it attains the normal size. The whole process is of distinct survival value; the animal cannot be permanently held by a limb. It is only the more prominent limbs which are likely to be held by a predator and thus autotomy is limited to the chelipeds and pereiopods. Some of the Astacura are known to pull off a damaged or crushed limb.

Autotomy is the voluntary shedding of body parts and has been observed in many arthropod taxa. It is most frequently employed to aid escape from passive adhesive traps such as those of Drosera.

Limb autotomy is most commonly employed by flies and harvestmen (order Opilionea). This strategy can be a highly effective defence mechanism against predators.

What is autotomy?

Autotomy is a form of defensive shedding that occurs in some arthropods when the limbs are caught by predators or other forces that threaten to damage or kill them. It involves shearing the limb off along its breakage plane. The limb is shed and may leave behind a stump of chitin, blood, or muscle. The stump enlarges after each molt until it returns to its normal size (Maginnis 2006a).

Shed limbs can be found in many invertebrates including reptiles, amphibians, fish, crustaceans and insects. However, this type of limb loss is more common in arthropods. This is because their skeletal system is exposed on the outside of their bodies. The limbs are also longer and thinner than most invertebrates, making them more likely to be grabbed by predators.

The rate of leg autotomy can be affected by predation attempts and complications with molting. One study found that 20% of the legs of individuals in a population of Didymuria violescens were shed after they reached maturity due to either molting complications or predation attempts.

Many lizards are capable of tail autotomy, which allows them to escape or distract predators if they get their tails grabbed. They can do this by creating a vertical fracture plane, containing no bone, passing through the body and part of the neural arch of each caudal vertebra. The fracture plane is absent in the cranial part of their tails to protect the hemipenes, fat deposits and other structures.

Limb autotomy can be very rapid ( 2 min sensu Emberts et al 2020) in some species of insect and arthropod, which is expected to help evade predators. However, it is more common for limbs to be lost during non-predatory entanglement, such as can occur during molting. In a controlled laboratory experiment, crickets that had undergone limb autotomy were less able to escape sticky traps of Drosera plants than those that had not.

How do arthropods evade predators?

Autotomy is a defensive behaviour in which animals voluntarily amputation an appendage such as a leg in order to improve their chances of escape from a predator (McVean 1982). It can be triggered by chemical, thermal and electrical stimulation but is perhaps most frequently initiated by mechanical stimulation during capture by a prey animal. Consequently, the amputation of an appendage is often viewed as a response to the risk of being crushed or otherwise damaged by the grip of a predator, although some arthropods also use limb autotomy for sexual defence (the specialized reproductive arm in male octopuses detaches from the body during mating).

Many invertebrates are capable of shedding their limbs, and this defensive behaviour can be invoked by a number of stimuli including vibrational communication, physical stress and molting complications. Limb autotomy is particularly common in insects and arthropods where it can be employed as a means of escaping from predators such as Drosera.

In a laboratory experiment, crickets and other arthropods capable of limb autotomy were shown to be able to shed their stuck legs in order to escape from Drosera’s sticky traps. Moreover, it was found that the ability to shed a leg is crucial in escaping from Drosera’s traps as the majority of captured individuals became fatally trapped unless they were able to escape by shedding their stuck limbs.

It has been demonstrated that limb autotomy can be employed to help arthropods evade a range of predators including birds, mammals and fish. In addition, the amputation of a limb is an effective defence against the crushing force of some vertebrates such as salamanders and amphibians.

In a study on the leg autotomy in the spur-legged phasmatid, Didymuria violescens, it was found that individuals with more legs lost to limb autotomy were less likely to escape from Drosera’s sticky capture. This is largely due to the fact that longer legs can keep the thorax and abdomen away from Drosera’s tentacles for an extended period of time, thus facilitating the escape mechanism.

Arthropod defence mechanism

Insects use a variety of strategies to evade predation, including autotomy. This behaviour may occur either for defensive purposes or as a result of complications associated with molting. Autotomy has also been used to test the effectiveness of various defence mechanisms. For example, if an insect’s legs get stuck to a sticky trap, it is likely that the animal will lose one or more of its legs by a process called self amputation in order to escape the trap (McVean 1982).

This ability to shed appendages can be beneficial because it reduces the chance that a predator mistakingly catches a leg instead of the body. It can also reduce the chances that a damaged or painful limb will be sucked into the organism’s digestive tract, which can cause considerable damage and even death.

The genus Didymuria is a good example of an arthropod that employs this defence mechanism. In a study, the authors found that the number of autotomised legs was negatively correlated with escape likelihood from adhesive mucilage traps. This is most likely due to the fact that the insects with multiple autotomised limbs were more likely to be trapped by their own entangled legs.

Another defensive strategy that arthropods use is the production of chemicals that are toxic or distasteful to predators. These chemicals are synthesised in the insect’s glandular tissue or sequestered from host plants and then released when the animal is under threat. The resulting toxins can be spread as a glandular ooze, into the air as a volatile pheromone or aimed as a spray directly at the predator.

Arthropods have a unique body structure that is unusual among vertebrates, in that it exhibits bilateral symmetry. This means that the left and right sides of the body are identical in size and arrangement of appendages, eyes and so on. As a result, these arthropods are very well camouflaged and can often go unnoticed by predators that are not familiar with the species.