Antennal sensilla of the stonefly Dinocras cephalotes (Plecoptera: Perlidae)

Sensory pathway in aquatic basal polyneoptera: Antennal sensilla and brain morphology in stoneflies

Aquatic insects represent a great portion of Arthropod diversity and the major fauna in inland waters. The sensory biology and neuroanatomy of these insects are, however, poorly investigated. This research aims to describe the antennal sensilla of nymphs of the stonefly Dinocras cephalotes using scanning electron microscopy and comparing them with the adult sensilla. Besides, central antennal pathways in nymphs and adults are investigated by neuron mass-tracing with tetramethylrhodamine, and their brain structures are visualized with an anti-synapsin antibody. No dramatic changes occur in the antennal sensilla during nymphal development, while antennal sensilla profoundly change from nymphs to adults when switching from an aquatic to an aerial lifestyle. However, similar brain structures are used in nymphs and adults to process diverging sensory information, perceived through different sensilla in water and air. These data provide valuable insights into the evolution of aquatic heterometabolous insects, maintaining a functional sensory system throughout development, including a distinct adaptation of the peripheral olfactory systems during the transition from detection of water-soluble chemicals to volatile compounds in the air. From a conservation biology perspective, the present data contribute to a better knowledge of the biology of stoneflies, which are very important bioindicators in rivers.

Flagellar sensilla of the hangingflies Bittacus planus Cheng and Bittacus sinicus Issiki (Mecoptera: Bittacidae)

Adult hangingflies are very sensitive to humidity, thereby frequently being used as ecological indicators to assess the degradation of the environment, especially forest ecosystems. Studies on the sense organs associated with hygro- and thermo-sensitivity, however, have been scant. Here, the ultramorphology and distribution of the flagellar sensilla were investigated in the adult hangingflies Bittacus planus Cheng, 1949 and Bittacus sinicus Issiki, 1931 using scanning electron microscopy. Four types of sensilla are identified in B. planus, including sensilla campaniformia, sensilla chaetica (SC1, SC2), sensilla basiconica (SB1, SB2), and sensilla coeloconica. In B. sinicus, sensilla chaetica III are present additionally from 7th to 14th flagellomeres. Abundant sensilla coeloconica are present on the flagella of Bittacus. Sensilla basiconica I are situated at the joints of flagellomeres. Sensilla campaniformia are densely distributed on the basal half of the first flagellomere, but scarcely on other flagellomeres. Sensilla chaetica II are present on the distal three flagellomeres. The size, abundance, and distribution of flagellar sensilla differ considerably between B. planus and B. sinicus. We infer that the abundance of sensilla coeloconica is likely associated with the hygro- and thermo-sensitivity in Bittacidae. Studies of flagellar sensilla can provide valuable information for subsequent electrophysiological, behavioral, biogeographical, and phylogenetic analyses of Bittacidae.

Gustation in insects: taste qualities and types of evidence used to show taste function of specific body parts

The insect equivalent of taste buds are gustatory sensilla, which have been found on mouthparts, pharynxes, antennae, legs, wings, and ovipositors. Most gustatory sensilla are uniporous, but not all apparently uniporous sensilla are gustatory. Among sensilla containing more than one neuron, a tubular body on one dendrite is also indicative of a taste sensillum, with the tubular body adding tactile function. But not all taste sensilla are also tactile. Additional morphological criteria are often used to recognize if a sensillum is gustatory. Further confirmation of such criteria by electrophysiological or behavioral evidence is needed. The five canonical taste qualities to which insects respond are sweet, bitter, sour, salty, and umami. But not all tastants that insects respond to easily fit in these taste qualities. Categories of insect tastants can be based not only on human taste perception, but also on whether the response is deterrent or appetitive and on chemical structure. Other compounds that at least some insects taste include, but are not limited to: water, fatty acids, metals, carbonation, RNA, ATP, pungent tastes as in horseradish, bacterial lipopolysaccharides, and contact pheromones. We propose that, for insects, taste be defined not only as a response to nonvolatiles but also be restricted to responses that are, or are thought to be, mediated by a sensillum. This restriction is useful because some of the receptor proteins in gustatory sensilla are also found elsewhere.

The First Study of Mating Mistakes in Stoneflies (Plecoptera) from China, with Remarks on Their Biological Implications

Currently, information on the biology of Plecoptera from China is scarce, particularly on mating behavior. In this paper, the existence of mating mistakes (erroneous mating attempts) involving 13 Chinese stonefly species (belonging to nine genera and three families) is reported. These erroneous mating behaviors can be included into three different categories: mating attempts between conspecific males (including the formation of erroneous mating balls), mating attempts between different taxa (including displacement attempts during copulation), and mating-related behaviors with non-living objects. From these behaviors, some aspects of stoneflies during mating, such as the physical competition between males, the sensorial mechanisms implied in triggering a mating behavior, the conditions favoring the mating mistakes, and the possible consequences of interspecific mating in the hybrid production, are discussed.

The Antennal Pathway of Dragonfly Nymphs, from Sensilla to the Brain

Dragonflies are hemimetabolous insects, switching from an aquatic life style as nymphs to aerial life as adults, confronted to different environmental cues. How sensory structures on the antennae and the brain regions processing the incoming information are adapted to the reception of fundamentally different sensory cues has not been investigated in hemimetabolous insects. Here we describe the antennal sensilla, the general brain structure, and the antennal sensory pathways in the last six nymphal instars of Libellula depressa, in comparison with earlier published data from adults, using scanning electron microscopy, and antennal receptor neuron and antennal lobe output neuron mass-tracing with tetramethylrhodamin. Brain structure was visualized with an anti-synapsin antibody. Differently from adults, the nymphal antennal flagellum harbors many mechanoreceptive sensilla, one olfactory, and two thermo-hygroreceptive sensilla at all investigated instars. The nymphal brain is very similar to the adult brain throughout development, despite the considerable differences in antennal sensilla and habitat. Like in adults, nymphal brains contain mushroom bodies lacking calyces and small aglomerular antennal lobes. Antennal fibers innervate the antennal lobe similar to adult brains and the gnathal ganglion more prominently than in adults. Similar brain structures are thus used in L. depressa nymphs and adults to process diverging sensory information.

Ovipositor setation in oldest insects (Insecta: Archaeognatha) revealed by SEM and He-ion microscopy

Archaeognatha represent the oldest living lineage of true insects (=Ectognatha), which are remarkable, among others, for plesiomorphic genital morphology and complicated mating behaviour. I used scanning electron microscopy and He-ion microscopy to examine the ovipositor morphology of seven species, in order to describe the cuticle microsculpture. The species studied are characterised by different types of the ovipositor setation pattern, which are considered an important taxonomic feature for Archaeognatha. The common and well discernible elements of ovipositor setation in Archaeognatha are: (1) non-articulated terminal seta, (2) grooved type I basiconic sensillum with apical pore, (3) multiporous type II basiconic sensillum, (4) articulated setae of different length. Coeloconica-like sensilla and campaniform sensilla demonstrate a variety of transient morphology. Results of this study provide morphological evidence of presence of olfactory receptors on the ovipositor in Archaeognatha. The possible functions of the ovipositor setation in Archaeognatha are discussed.

Antennal responses to volatile organic compounds in a stonefly

Notwithstanding their long antennae, especially in the adult stage, Plecoptera represent a poorly studied group as regards their sensory structures and their ability to perceive olfactory stimuli is so far totally unknown. A recent investigation on the antenna of Dinocras cephalotes revealed two kinds of putative olfactory sensilla (multiporous single walled sensilla and digitated double walled sensilla). The present electrophysiological study (electroantennography, EAG), in which male and female antennae of D. cephalotes (Plecoptera, Perlidae) have been stimulated with a set of generic odours belonging to different functional groups, shows that Plecoptera can perceive olfactory cues through their antennal sensilla. Indeed, although many chemicals did not elicit any response, high EAG activity has been recorded in response to pentanoic acid, propanal, butyric acid, propionic acid, isoamylamine and ammonia. The response was very similar in both sexes even if propanal elicited a response only in males. EAG dose-responses curves in both males and females showed that EAG responses were similar in males and females and generally increased in amplitude with increasing doses of the chemicals. Behavioural, neuroanatomical and molecular investigations on Plecoptera olfaction, could be particularly interesting not only to increase the knowledge of the adult stonefly behaviour but also to better understand the adaptation of the olfactory sensory system in aquatic insects moving from land to water.