Frictional surfaces of the elytra-to-body arresting mechanism in tenebrionid beetles (Coleoptera : Tenebrionidae) : design of co-opted fields of microtrichia and cuticle ultrastructure

In beetles, the system responsible for an attachment of forewings (elytra) to the thorax consists of interlocking fields of microtrichia (MT) located between thorax and body and between left and right elytra. The present study provides comparative data about microtrichia design on the thorax and elytra in three species of tenebrionid beetles (Tribolium castaneum, Tenebrio molitor, Zophobas rugipes) (Coleoptera : Tenebrionidae), which are different in their size. The length, width, density and directionality of microtrichia in 13 MT fields (4 on the thorax, 1 on the abdomen, 7 on the elytra, and 1 on the costal vein of the hindwing) were quantified. (1) Parameters studied are dependent on the dimension of an insect. The length of the microtrichia of most fields compared increases with an increase in body size. The MT width in the majority of fields increases with an increase in the elytra length. The MT density decreases with an increase in the elytra length. (2) Both width and length of microtrichia increase with an increase in the distance between single MT. The density of outgrowths increases with an increase in their length and width. (3) The fields oriented along the same spatial axis constitute functional groups responsible for a particular direction. Co-opted fields can be oriented in the same or opposite directions. (4) The design of MT correlates in co-opted surfaces. There are 3 field groups, which were stated as functionally corresponding to one another : the medial, anterio-lateral, and posterio-lateral. The lengths and widths of microtrichia from fields of these functional groups were quite similar in corresponding fields. Length-to-width ratios of MT in elytral fields were usually weakly correlated with those of thoracic fields. The distances between microtrichia on the elytra surface directly depended on those of the thorax. Distance-to-width ratio of MT of one surface slightly increased with an increase in this parameter on the co-opted surface. The MT densities on co-opted fields were usually quite different. (5) The ultrastructure of the cuticle suggests differences in the material properties of the cuticle between MT fields. The thoracic fields usually consist of elastic cuticle, whereas elytral fields are much harder. Usually, a MT field of elastic cuticle corresponds to the field composed of hard cuticles. The study also provides information about the ultrastructure of epidermal cells and about the design of pore channels, which are presumably responsible for production and transport of an adhesive secretion into the area of contact between lateral fields. Sensory organs monitoring contact between co-opted binding sites were also studied. The results of this study may aid in understanding the morphological basis of cuticular microsculptures acting as frictional devices.