Horst Hertel

The spectral input systems of hymenopteran insects and their receptor-based colour vision.

SummarySpectral sensitivity functions S(λ) of single photoreceptor cells in 43 different hymenopteran species were measured intracellularly with the fast spectral scan method. The distribution of maximal sensitivity values (λmax) shows 3 major peaks at 340 nm, 430 nm and 535 nm and a small peak at 600 nm. Predictions about the colour vision systems of the different hymenopteran species are derived from the spectral sensitivities by application of a receptor model of colour vision and a model of two colour opponent channels. Most of the species have a trichromatic colour vision system. Although the S(λ) functions are quite similar, the predicted colour discriminability curves differ in their relative height of best discriminability in the UV-blue or bluegreen area of the spectrum, indicating that relatively small differences in the S(λ) functions may have considerable effects on colour discriminability. Four of the hymenopteran insects tested contain an additional R-receptor with maximal sensitivity around 600 nm. The R-receptor of the solitary bee Callonychium petuniae is based on a pigment (P596) with a long λmax, whereas in the sawfly Tenthredo campestris the G-receptor appears to act as filter to a pigment (P570), shifting its λmax value to a longer wavelength and narrowing its bandwidth. Evolutionary and life history constraints (e.g. phylogenetic relatedness, social or solitary life, general or specialized feeding behaviour) appear to have no effect on the S(λ) functions. The only effect is found in UV receptors, for which λmax values at longer wavelengths are found in bees flying predominantly within the forest.

Degradation of Lignin Monomers by the Hindgut Flora of Xylophagous Termites

Summary The hindgut flora of the lower termites Mastotermes darwiniensis (Froggatt) and Reticulitermes santonensis (Feytaud) and the higher termite Nasutitermes nigriceps (Haldeman) was tested for its in vivo and in vitro capability to degrade lignin monomers and related aromatic compounds. From the three termites, 53 bacteria were isolated in liquid and on solid media, containing aromatic compounds as carbon source. Most of the tested aromatic compounds were degraded aerobically by mixed and pure cultures. Under fermentative conditions the aromatic compounds were only partially degraded and the aromatic ring was not split. The results suggest that in the anaerobic hindgut the breakdown of aromatic ring systems requires oxygen, which is most probably supplied via the aerated paunch epithelium.

Repellent and toxic effects of plant extracts on subterranean termites (Isoptera: Rhinotermitidae)

Abstract The effects of four plant extract formulations on the orientation and survival of subterranean termites were tested. In choice experiments, extract-treated filter paper had a significantly repellent effect on groups of Reticulitermes santonensis De Feytaud, R. virginicus (Banks), Coptotermes formosanus Shiraki, and Schedorhinotermes intermedius Breinli. There was no species-specific difference in avoidance behavior toward the tested concentrations. No-choice experiments revealed toxic properties of all investigated extracts by contact or airborne compounds against R. santonensis. However, high termite mortality was only achieved by forced direct or forced indirect exposure to the plant material. Feeding deterrence or digestive toxicity could not be judged in these experiments. One of the extracts was efficiently used for soil treatments to protect a food substrate against R. santonensis infestation. Extract-treated barriers in the experiments did not affect mortality compared with control trials but prevented termites from penetrating treated soil.

Systematic search for food in the subterranean termite Reticulitermes santonensis De Feytaud (Isoptera, Rhinotermitidae)

Summary: The search for food in subterranean termites is investigated in the French species Reticulitermes santonensis De Feytaud (Isoptera, Rhinotermitidae). Not accompanied by soldiers, only few pioneer workers explore a new territory systematically in every direction by means of short exploratory trails (median: 1.3 cm). Following these trails, termites build subterranean tunnels and/or galleries in the open, hereby forming an extensive regularly branched net. Food is detected over distance by perceiving volatiles emanating from the wood. Then foragers appear more frequently and undertake longer exploratory runs (up to 15 cm), which are directed towards the odour source. Following contact with the wood, sucessful workers return to the nest precisely along their exploratory trail and hereby lay a recruitment trail. This recruitment trail is immediately followed by further workers as well as by single soldiers. Following the course of this trail, termites build a non‐branched subterranean tunnel or a gallery above ground leading straight to the wood. This direct access requires less investment and provides workers with a shelter ensuring a rapid and safe exploitation of the food source.

Change of synapse frequency in certain photoreceptors of the honeybee after chromatic deprivation

SummaryBees reared under UV-light are less sensitive to long wavelength light in phototaxis experiments than are control bees. Electroretinogram measurements indicate that selective reduction in spectral sensitivity is related to smaller lamina potentials to green light. Histological investigation of synapse frequency of green sensitive photoreceptor terminals reveal a decrease in number of synapses due to the selective wavelength deprivation.

FEEDING STIMULATING SIGNAL IN LABIAL GLAND SECRETION OF THE SUBTERRANEAN TERMITE Reticulitermes santonensis

The paired labial glands of the French subterranean termite Reticulitermes santonensis Feytaud are located in the thorax. In the head, the glandular ducts join with those of the water sacs. In feeding choice tests with two semicircles of moist filter paper as food, workers of R. santonensis preferred the semicircle treated with labial gland extract compared to the semicircle treated with water (control). The labial gland secretion carries a signal that stimulates gnawing and feeding by termite workers during food exploitation. The extract of the labial gland even elicited feeding behavior when applied without food (on glass plates). The content of the water sacs was not effective as a feeding stimulus and neither were different body parts except for the fat-body. Water sacs are thus not a reservoir for the labial gland secretion, but their contents may serve as a solvent or carrier for the phagostimulant signal. The signal is highly polar, heat-resistant, nonvolatile, and thus very persistent. There is evidence that the signal from the labial gland is not species specific. Instead, it might be part of a general strategy by termites to exploit food sources.

Isolation and ultrastructural features of a new strain of Dimastigella trypaniformis Sandon 1928 (Bodonina, Kinetoplastida) and comparison with a previously isolated strain

A strain of the free-living flagellate Dimastigella trypaniformis Sandon was isolated from the gut contents of the Australian termite Mastotermes darwiniensis Froggatt. The fine structure of the isolate (Ulm strain) was compared with a strain isolated from soil in Scotland (Glasgow strain). The biflagellated trophozoites possess great morphological similarities with the bodonids and cryptobiids such as a polykinetoplastic mitochondrial complex, a flagellar paraxial rod, and a cytopharyngeal complex. However, characteristic features are also found such as the two opposed basal bodies and the two flagella, one anteriorly and the other recurrently directed, the latter adhering to a ventral furrow, a rostrum as well as a cytostome and a cytopharynx with rostral structures. These organisms have a typical motion with a highly flexible cell body. They are able to form resistant cysts in the culture. The two strains are distinguishable by small differences in their growth characteristics in different media and qualitative ultrastructural features.

Response properties of stained monopolar cells in the honeybee lamina

Summary1.Monopolar cells of the first visual ganglion, the lamina, of the bee were recorded from and stained intracellularly.2.Several different response types to pulses of spectral light were found. The most common response type hyperpolarized in a phasic-tonic fashion. The tonic hyperpolarizing response frequently decreased gradually, but in some cases increased with lasting illumination. Some cells also gave a transient response to light-OFF. In contrast, one stained and several unstained cells showed depolarizing responses. Five cells exhibited spiking responses under normal physiological conditions.3.The V/log I-functions were steeper than those of the photoreceptors and, in some cases, had both rising and falling parts with increasing intensities. The spectral sensitivity obtained with the constant response method showed a peak in the green (510–535 nm) in most cells. A series of spectral flashes revealed an additional type with highest sensitivity in UV. Indirect evidence was found in one cell for spectral opponent processing.4.Two morphological types of monopolar cells were stained. These correspond well to Ribis (1976) L1 and L2 cells, with some differences in detail. The most frequently stained cell type closely resembles his L2 type. All 3 stained spiking cells were of this type.

Centrioles to basal bodies in the spermiogenesis of Mastotermes darwiniensis (Insecta, Isoptera).

In addition to their role in centrosome organization, the centrioles have another distinct function as basal bodies for the formation of cilia and flagella. Centriole duplication has been reported to require two alternate assembly pathways: template or de novo. Since spermiogenesis in the termite Mastotermes darwiniensis lead to the formation of multiflagellate sperm, this process represents a useful model system in which to follow basal body formation and flagella assembly. We present evidence of a possible de novo pathway for basal body formation in the differentiating germ cell. This cell also contains typical centrosomal proteins, such as centrosomin, pericentrin-like protein, gamma-tubulin, that undergo redistribution as spermatid differentiation proceeds. The spermatid centrioles are long structures formed by nine doublet rather than triplet microtubules provided with short projections extending towards the surrounding cytoplasm and with links between doublets. The sperm basal bodies are aligned in parallel beneath the nucleus. They consist of long regions close to the nucleus showing nine doublets in a cartwheel array devoid of any projections; on the contrary, the short region close to the plasma membrane, where the sperm flagella emerge, is characterized by projections similar to those observed in the centrioles linking the basal body to the plasma membrane. It is hypothesized that this appearance is in connection with the centriole elongation and further with the flagellar axonemal organization. Microtubule doublets of sperm flagellar axonemes are provided with outer dynein arms, while inner arms are rarely visible.

Cerebral extraocular photoreceptors in beetles

Cell clusters containing rhabdomeric structures and screening pigments are located in the optic lobes of pupae and adult beetles of Tenebrio molitor and Hylotrupes bajulus. Due to their resemblance with the compound eyes, the cell clusters were called cerebral extraocular photoreceptors (CEPs) and can readily be detected macroscopically as black spots spread between the lamina and medulla. CEPs of T. molitor responded to bright light with a transient depolarisation followed by a sustained plateau. In contrast, CEPs of H. bajulus responded to bright light either with small depolarisations or did not respond to the range of light intensities presented. CEP cells labelled with Lucifer yellow had axons projecting toward the medullar region. The maximum spectral sensitivity of CEPs was broadly tuned to the green region (λmax at 524 nm). The role of CEPs in insects is still a matter of wide speculation, due to the lack of consistent systematic behavioral studies about insects carrying photoreactive CEPs. Here we suggest that the scattered distribution of CEPs in the optic lobes could allow a temporal pooling of information about the surrounding illumination, thereby entraining some rhythmic photobehavioural responses.