Type, distribution and quantitative characteristics of sensory organs on the larval head of Tomicus brevipilosus (Coleoptera: Curculionidae: Scolytinae)

Objective To provide a scientific basis for further exploration of foraging and moving about the larval behaviors of Tomicus bravipilosus (Coleoptera: Curculionidae: Scolytinae), we investigated the morphological adaptation and function of the sensilla on the larval head of this bark beetle.

Method We examined and compared the types, distribution, and abundance of sensory organs on the head of the first, second, and third instar larvae using scanning electron microscopy (SEM).

Result The head shape of the larvae was hypognathous. Its head appendages contained a pair of antennae, stemmata, mandible, maxilla, maxillary palp, labial palp, as well as a single labrum and labium. Totally, there were 15 types of identified sensilla. The reduced antennae with one segment had five types of antennal sensilla, identified as sensilla basiconica type Ⅰ and sensilla twig basiconica type Ⅰ−Ⅳ. The biting mouthparts were involved in 12 types, including sensilla chaetica type Ⅰ−Ⅴ, sensilla trichodea type Ⅰ−Ⅱ, sensilla basiconica type Ⅱ, sensilla digitiformina, and sensilla twig basiconica type Ⅱ, Ⅳ, and Ⅴ. Sensilla trichodea and sensilla chaetica having smooth and nonporous surface were widespread and classified as mechanoreceptors. Sensilla basiconica and sensilla twig basiconica having the surface pores distributed exclusively on the apex of the antennae, maxillary palp and labial palp, and were classical chemoreceptors. Sensilla digitiformia having only one unit on the second segment of every maxillary palp were considered as the acoustic sensor for the mechanical vibrations. With an increase of larval instar, each sensilla types, abundance, and distribution on the head appendages kept consistent with the different instar larvae. However, sensilla sizes showed exponential growth compared with the last instar larvae.

Conclusion This research clarifies the head structures and sensilla types, distribution, abundance, and instar changes of the sensilla on the larval head of T. brevipilosus. Chemical sensors are concentrated at the tips of the antennae, mandibular palps, and labial palps, which is a distribution pattern that aids the larvae in feeding and moving within the bark-cavities. The study has conducted a detailed exploration of the functions of the chemical sensors, which will provide a scientific basis for the next step of studying the information perception of T. brevipilosus larvae within its galleries. It is expected to take a shred of scientific evidence for further behavioral drivers of chemical sensation in the gallery of T. brevipilosus larvae.