The full bristles and the sensory bristles (sensilla) play several roles, respectively that of protection or ornament and for the innervated bristles, the role of mechanical receiver but also for some of them that of chemoreceptor. In order to better understand the functional difference between a simple silk and an innervated silk, here are two diagrams:
The first diagram (on the right) shows the typical layout of a solid silk. With no sensory cells, it plays a special role in springtail in the protection against water and more particularly the risk of asphyxia (see The cuticle).
The sensilla, in turn, are distinguished from full bristles (or simple) because they have at their base a nerve cell that allows the detection of voltage variations or vibrations transmitted by touch and air or liquid flows. This nerve ending also allows in some cases the senses to be able to be tasty or olfactory. The taste bristles are generally located on the mouthparts and olfactory bristles opening through pores (photo below right) located more particularly at the antennas.
The second diagram (left) shows the basic structure of a tactile sensilla in arthropods.
It is composed of a sensory cell whose dendritic end penetrates into the cuticular canal of the secreted silk by an adjoining cell, called trichogene which surrounds it and which, itself, is separated from the rest of the integument by another "tormogenic" auxiliary cell.
The sensilla can present various forms from the elongated silk exceeding largely the cuticle, until the button just flush or pressed into a small crypt (photo below right). These tactile sensilla are found on a large part of the integument and are particularly abundant on the antennae, tarsi and cerques.
The interaction between the silk and the end of the sensory cell is favored by the dendritic structure of the latter, rich in microtubules associated with the membrane which is in contact with the silk, so that any movement of the latter causes distension. capable of inducing a potential directly transmitted in the form of an electrical impulse to the central nervous system1.
It should be noted that the sensilla, in addition to their mechano-receptive capacities, are distinguished according to two types, taste and olfactory sensilla disposed respectively on the oral apparatus and at the level of the antennas. The latter are able to detect the chemical molecules that penetrate the stem of the silk through the fine pores with which it is endowed.
The in-depth study of the different silks of springtails shows that some can specialize to perceive various stimuli (chemical, mechanical ...). The researchers have listed four types: microsetae (which uniformly cover the body), scales (which are flattened bristles), macrosetae (morphologically differentiated, less numerous and distributed specifically) and trichobothroses (mechanoreceptives, long and thin) .
In these illustrations, we can see the lateral implantation of mechanical-receptive bristles (trichobothria) on Symphypleones and Neelipleones (see below: Sminthurides malmgreni, below, from left to right: Sminthurinus reticulatus, Neelus murinus, Sminthurides malmgreni and Sminthurinus aureus).
These bristles are able to detect minute drafts or vibrations from all directions. The nerve impulses then generated by the sensory cells (in green on the second diagram of this article) allow the springtail to have information on possible changes of states or presences in its proximity. Frans Janssens (collembola.org) compares the functioning of trichobothries to that of a joystick capable of perceiving and retransmitting solicitations / information from all directions.
The bristles also have a wide range of shapes. Thus we find straight, curved or "s" shaped, but also with sharp points, flattened, forked or branched (comparable to a feather duster). They may be larger at their base or conversely have larger shapes near their ends (like an elongated club). Their distribution and density are also variable and are the subject of studies to characterize the species. We can therefore consider that the silks of springtails, through their wide morphological palette and their various abilities (which are found just as operational as in their distant cousins, insects and arachnids) play a major role in their sensory potential, probably much more important than their vision, whose organs (ocelli / ocular plaques) are generally underdeveloped, atrophied or even absent.
1: Elements of the beginning of the article and adapted figures of: Animal Physiology - Raymond Gilles. Publisher: De Boek & Larcier s.a. Brussels (2006)
Photo montage © Philippe Garcelon.