Comparison of cleaning methods for delicate insect specimens for scanning electron microscopy.

The objective of the present study was to compare cleaning methods for delicate insect specimens for investigations with scanning electron microscopy (SEM). As typical specimens we used aquatic larvae of mosquitoes, springtails, larvae of mayflies and caterpillars because they are very fragile and large parts of their body consist of soft tissue. Additionally their cuticle is very often covered with dirt, soil particles or other materials. Cleaning with ultrasonic sound, as the most common cleaning method used for SEM, will destroy fragile insects. Therefore we tested different procedures to remove the dirt particles. In a first approach we compared cleaning with Potassium hydroxide (KOH), Proteinase K, and Triton X in aquatic larvae of flies, which were available in numbers and kept under the same conditions. As our results showed that the treatment with KOH gives the best results we treated in a second approach springtails, larvae of mayflies and caterpillars only with KOH. The springtails and caterpillars were largely free of particles after treatment with KOH; however, the larvae of mayflies were still covered with remnants of diatoms and precipitates of calcium carbonate of the algae. KOH dissolves organic impurities, on the other hand silicon dioxide and lime crusts are not solved. With this limitation, treatment with KOH is a simple technique for routine use as cleaning method for fragile insect specimens for SEM.

The larval head of Exechia (Mycetophilidae) and Bibio (Bibionidae) (Diptera).

Exechia and Bibio have retained several plesiomorphic groundplan features of Diptera and Bibionomorpha, including a fully exposed and sclerotized head capsule, the transverse undivided labrum, the absence of movable premandibles, and undivided mandibles without combs. The fusion of the hypostomal bridge with the head capsule and largely reduced antennae are derived features shared by both taxa. The absence of teeth at the anterior hypostomal margin is a potential autapomorphy of Bibionomorpha. A basal position of Anisopodidae is suggested by a number of plesiomorphies retained in this family. Apomorphies of Bibionomorpha excluding Anisopodidae are the reduction of tentorial elements, the partial fusion of the labrum and clypeus, one-segmented antennae, the absence of a separate submental sclerite, the loss of the labial palpus, and the reduction of the pharyngeal filter apparatus. Head structures of Bibio are largely unmodified. The subprognathous orientation is one of few autapomorphic features. In contrast, the mouthparts of Exechia are highly modified in correlation with the specialized food uptake. The rasping counterrotating movements of maxillae and mandibles with teeth oriented in opposite directions are carried out by strongly developed extensors and flexors of the paired mouthparts. The modified labium mechanically supports the "drill head" formed by the mandibles und maxillae. The necessary stability of the head capsule is provided by the hypostomal bridge which also compensates the far-reaching reduction of the tentorium.

dOr83b--receptor or ion channel?

Odorant signals are detected by binding of odor molecules to odorant receptors. These belong to the G protein-coupled receptor family. They in turn couple to G proteins, most of which induce cAMP production. This second messenger activates ion channels to depolarize the olfactory sensory neuron, thus providing a signal for further neuronal processing. Recent findings challenge this concept of olfactory signal transduction in insects, since their odorant receptors, which lack any sequence similarity to other G protein-coupled receptors, are composed of conventional odorant receptors (e.g., Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure similar to G protein-coupled receptors, but has an inverted orientation in the plasma membrane. Still, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. In our experiments we demonstrated that application of odorants to mammalian cells co-expressing Or22a and Or83b results in nonselective cation currents activated via both an ionotropic and a metabotropic pathway, and a subsequent increase in the intracellular Ca(2+) concentration. Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic nucleotide-activated nonselective cation channels.

Drosophila odorant receptors are both ligand-gated and cyclic-nucleotide-activated cation channels.

From worm to man, many odorant signals are perceived by the binding of volatile ligands to odorant receptors that belong to the G-protein-coupled receptor (GPCR) family. They couple to heterotrimeric G-proteins, most of which induce cAMP production. This second messenger then activates cyclic-nucleotide-gated ion channels to depolarize the olfactory receptor neuron, thus providing a signal for further neuronal processing. Recent findings, however, have challenged this concept of odorant signal transduction in insects, because their odorant receptors, which lack any sequence similarity to other GPCRs, are composed of conventional odorant receptors (for example, Or22a), dimerized with a ubiquitously expressed chaperone protein, such as Or83b in Drosophila. Or83b has a structure akin to GPCRs, but has an inverted orientation in the plasma membrane. However, G proteins are expressed in insect olfactory receptor neurons, and olfactory perception is modified by mutations affecting the cAMP transduction pathway. Here we show that application of odorants to mammalian cells co-expressing Or22a and Or83b results in non-selective cation currents activated by means of an ionotropic and a metabotropic pathway, and a subsequent increase in the intracellular Ca(2+) concentration. Expression of Or83b alone leads to functional ion channels not directly responding to odorants, but being directly activated by intracellular cAMP or cGMP. Insect odorant receptors thus form ligand-gated channels as well as complexes of odorant-sensing units and cyclic-nucleotide-activated non-selective cation channels. Thereby, they provide rapid and transient as well as sensitive and prolonged odorant signalling.