Lehr's fields of campaniform sensilla in beetles (Coleoptera): Functional morphology. I. General part and allometry

In this first of three articles we show the construction of the articular part of the elytron, the root. The root bears a conspicuous field of campaniform sensilla. This field was studied using light and scanning electron microscopes. The diversity of shape of the field among beetles, types of orientation of elongated sensilla within the field, individual variability of their number among conspecifics are demonstrated. Elongated sensilla point to the junction of the elytron with the second axillary plate. Presumably, they monitor twist movement in this junction, which is possible if the elytron is open. The goal of the whole project is to reveal the effect of both structure and function of the hind wings and elytra on the morphology of this mechanosensory field. Our data on allometric relationships between the animal size and quantitative characteristics of the field in normally flying beetles provide an important background for further functional analysis of this sensory organ.We selected 14 series of several species belonging to the same taxon but differing in size from big to small. It is revealed that the area of the sensory field is directly proportional to the elytral area, whereas the number of sensilla is proportional to the square root of the elytral area. Despite the great range in the elytral area (1500 times) in series of selected species the area of an external pit or cap of a single sensillum varies only 25-fold. The density of sensilla per unit area of the sensory field increases with decrease of the elytral area.     

Lehr's fields of campaniform sensilla in beetles (Coleoptera): Functional morphology. III. Modification of elytral mobility or shape in flying beetles

Some flying beetles have peculiar functional properties of their elytra, if compared with the vast majority of beetles. A “typical” beetle covers its pterothorax and the abdomen from above with closed elytra and links closed elytra together along the sutural edges. In the open state during flight, the sutural edges diverge much more than by 90°. Several beetles of unrelated taxa spread wings through lateral incisions on the elytra and turn the elytron during opening about 10–12° (Cetoniini, Scarabaeus, Gymnopleurus) or elevate their elytra without partition (Sisyphus, Tragocerus). The number of campaniform sensilla in their elytral sensory field is diminished in comparison with beetles of closely related taxa lacking that incision. Elytra are very short in rove beetles and in long-horn beetles Necydalini. The abundance of sensilla in brachyelytrous long-horn beetles Necydalini does not decrease in comparison with macroelytrous Cerambycinae. Strong reduction of the sensory field was found in brachyelytrous Staphylinidae. Lastly, there are beetles lacking the linkage of the elytra down the sutural edge (stenoelytry). Effects of stenoelytry were also not uniform: Oedemera and flying Meloidae have the normal amount of sensilla with respect to their body size, whereas the sensory field in the stenoelytrous Eulosia bombyliformis is 5–6 times less than in chafers of the same size but with normally linking broad elytra.     

Numerical investigation of the aerodynamic characteristics of a hovering Coleopteran insect

The aerodynamic characteristics of the Coleopteran beetle species Epilachna quadricollis, a species with flexible hind wings and stiff elytra (fore wings), are investigated in terms of hovering flight. The flapping wing kinematics of the Coleopteran insect are modeled through experimental observations with a digital high-speed camera and curve fitting from an ideal harmonic kinematics model. This model numerically simulates flight by estimating a cross section of the wing as a two-dimensional elliptical plane. There is currently no detailed study on the role of the elytron or how the elytron-hind wing interaction affects aerodynamic performance. In the case of hovering flight, the relatively small vertical or horizontal forces generated by the elytron suggest that the elytron makes no significant contribution to aerodynamic force.     

Flow Visualization of Rhinoceros Beetle (Trypoxylus dichotomus) in Free Flight

Aerodynamic characteristics of the beetle,Trypoxylus dichotomus,which has a pair of elytra (forewings) and flexible hind wings,are investigated.Visualization experiments were conducted for various flight conditions of a beetle,Trypoxylus dichotomus:free,tethered,hovering,forward and climbing flights.Leading edge,trailing edge and tip vortices on both wings were observed clearly.The leading edge vortex was stable and remained on the top surface of the elytron for a wide interval during the downstroke of free forward flight.Hence,the elytron may have a considerable role in lift force generation of the beetle.In addition,we reveal a suction phenomenon between the gaps of the hind wing and the elytron in upstroke that may improve the positive lift force on the hind wing.We also found the reverse clap-fling mechanism of the T.dichotomus beetle in hovering flight.The hind wings touch together at the beginning of the upstroke.The vortex generation,shedding and interaction give a better understanding of the detailed aerodynamic mechanism of beetle flight.     

Effect of temperature and diet on hind wing colouration development and elytral hardness of adult Colorado potato beetle (Coleoptera: Chrysomelidae)

The effects of food and temperature on the development of colour pigment in the hind wings of adult Colorado potato beetle, Leptinotarsa decemlineata (Say), were investigated. In a replicated study, adults were held at 18°C, 28°C and 18/28°C on potato foliage (Solanum tuberosum L.), potato tubers, or without food in controlled-environment chambers. Representative subsamples of wings were collected at two-day intervals, mounted on microscope slides, and photographed to document the progression of colour change. Observations were also made on elytral hardening over time. Hind wing colour developed more quickly at 28°C than at 18°C, and after three weeks had attained a deeper red colour at the higher temperature. Colour development was also more rapid when adult beetles were fed on foliage compared with tubers. In foliage-fed beetles, elytra hardened more quickly at 28°C than 18°C, and many tuber-fed beetles never developed hardened elytra, regardless of temperature treatment. Unfed beetles developed no hind wing colour pigment and their elytra remained soft for the duration of the experiment. Colour plates documenting wing colour development over time are presented; variation in colour development under the conditions tested, suggests that the use of hind wing colouration to estimate beetle age in the field may be problematic.     

Neural projection patterns from homeotic tissue of Drosophila studied in bithorax mutants and mosaics.

The central projection patterns of sensory cells from the wing and haltere of Drosophila, as revealed by filling their axons with cobalt, consist of dorsal components arising from small campaniform sensilla and ventral components arising from large campaniform sensilla and from bristles. All of the bristles of the wing are innervated, some singly and some multiply. All three classes of sensilla are strongly represented on the wing, but the haltere carries primarily small campaniform sensilla and has a correspondingly minute ventral projection. In bithorax mutants in which the haltere is transformed into wing, ventral components are added to the projection pattern, while the dorsal components appear as if haltere tissue were still present. Thus, the three classes of receptors not only produce different projection patterns when they develop in their native mesothoracic segment, but also behave differently in the homeotic situation. Consequently, different developmental programs are inferred for each class. When somatic recombination clones of bithorax tissue are generated in phenotypically wild-type flies, they also produce ventral projections. However, these projections of mutant fibers into wild-type ganglia differ in certain details from the projections of mutant fibers into mutant ganglia. Thus, homeotic changes are inferred to occur in the CNS of mutant flies, but these are not required for the execution of those developmental instructions carried in the genome of large campaniform and bristle sensory cells which specify that their axons should grow ventrad in the CNS.     

Indirect closing of elytra by the prothorax in beetles (Coleoptera): general observations and exceptions

Voluntary movements of the prothorax and the elytra in tethered flying beetles and manually induced movements of these parts in fresh dead beetles were recorded in 30 species representing 14 families. Participation of prothoracic elevation in the closing of the elytra was demonstrated in three ways. (i) The elevation was always simultaneous with elytral closing, in contrast to depression and elytral opening; a rare exception occurred in Lucanus cervus, whose elytra sometimes started to close before the cessation of wing strokes and the elevation of the prothorax. (ii) The manipulated elevation always induced closing of the spread elytra; the mechanical interaction between the hind edge of the pronotum and the roots of the elytra is a universal mechanism of closing the elytra in beetles. (iii) The prevention of pronoto-elytral contact in live beetles by the excision of the hind edge of the pronotum in front of the root prevented elytral closing after normal flight. Exceptions to this rule included some beetles that were able to close their elytra after such an excision: tiger beetles and diving beetles (seldomly) and rose chafers (always). This ability in Adephaga may be explained by attachments of the muscle actuating the 4th axillary plate, which differ from the attachments in Polyphaga. Cetoniinae open their elytra only by a small amount. It is proposed that their small direct adductors in combination with the elasticity of the sclerites are enough to achieve elytral closing without additional help from the prothorax.     

On the Axillary Sclerites and Their Role in the Mechanism of Flexion and Extension of the Wings in the Indian Field Cricket Gryllus bimaculatus Deg. (Gryllidae,Orthoptera)

A detailed account of the structure of the axillary sclerites of the fore and the hind wings is given. In the fore wing the first axillary is less developed than the second. In the hind wing the second axillary is less developed than the first. The inverse relative development of the first and the second axillary sclerites in the fore and the hind wings has arisen in response to the different type of role they have to play in the two wings. The structure of axillaries reveals that their modifications are intimately associated with the efficient mechanism of flexion and extension of the wings.     

Haltere morphology and campaniform sensilla arrangement across Diptera

One of the primary specializations of true flies (order Diptera) is the modification of the hind wings into club-shaped halteres. Halteres are complex mechanosensory structures that provide sensory feedback essential for stable flight control via an array of campaniform sensilla at the haltere base. The morphology of these sensilla has previously been described in a small number of dipteran species, but little is known about how they vary across fly taxa. Using a synoptic set of specimens representing 42 families from all of the major infraorders of Diptera, we used scanning electron microscopy to map the gross and fine structures of halteres, including sensillum shape and arrangement. We found that several features of haltere morphology correspond with dipteran phylogeny: Schizophora generally have smaller halteres with stereotyped and highly organized sensilla compared to nematoceran flies. We also found a previously undocumented high variation of haltere sensillum shape in nematoceran dipterans, as well as the absence of a dorsal sensillum field in multiple families. Overall, variation in haltere sensillar morphology across the dipteran phylogeny provides insight into the evolution of a highly specialized proprioceptive organ and a basis for future studies on haltere sensory function.     

Proprioceptor organs in Cicadella viridis (L.) (Homoptera : Cicadellidae)

The external proprioceptor organs of Cicadella viridis (L.) (Homoptera: Cicadellidae) are identified and localized. They are composed of hair and campaniform sensilla grouped together and located in or near the joint areas of various parts of the body or in cuticular areas, which come into contact with moving parts. The hair sensilla, 8–75 μm long, are arranged in hair plates or in hair rows; they detect relative movement between parts of the body. Proprioceptor organs of this kind, localized in proximity to joint areas, were found on the legs and on the abdomen. Proprioceptor organs composed of hair sensilla located far from the articular areas were found on the 3rd pair of legs, the thorax, the hind wings, and the abdomen. The campaniform sensilla, which are arranged in fields and groups, in cuticular areas subject to deformations, are of 2 types: with raised caps (type I) and with flat caps (type II). Organs with campaniform sensilla were found on the legs, thorax and wings.     

How Could Beetle＇s Elytra Support Their Own Weight during Forward Flight？

The aerodynamic role of the elytra during a beetle＇s flapping motion is not well-elucidated, although it is well-recognized that the evolution of elytra has been a key in the success of coleopteran insects due to their protective function. An experimental study on wing kinematics reveals that for almost concurrent flapping with the hind wings, the flapping angle of the elytra is 5 times smaller than that of the hind wings. Then, we explore the aerodynamic forces on elytra in free forward flight with and without an effect of elytron-hind wing interaction by three-dimensional numerical simulation. The numerical results show that vertical force generated by the elytra without interaction is not sufficient to support even its own weight. However, the elytron-hind wing interaction improves the vertical force on the elytra up to 80%; thus, the total vertical force could fully support its own weight. The interaction slightly increases the vertical force on the hind wind by 6% as well.     

Flexible Wing Kinematics of a Free-Flying Beetle (Rhinoceros Beetle Trypoxylus Dichotomus)

Detailed 3-Dimensional (3D) wing kinematics was experimentally presented in free flight of a beetle,Trypoxylus dichotomus,which has a pair of elytra (forewings) and flexible hind wings.The kinematic parameters such as the wing tip trajectory,angle of attack and camber deformation were obtained from a 3D reconstruction technique that involves the use of two synchronized high-speed cameras to digitize various points marked on the wings.Our data showed outstanding characteristics of deformation and flexibility of the beetle's hind wing compared with other measured insects,especially in the chordwise and spanwise directions during flapping motion.The hind wing produced 16％ maximum positive camber deformation during the downstroke.It also experienced twisted shape showing large variation of the angle of attack from the root to the tip during the upstroke.     

Thoracic mechanoreceptors in the wing bases of Heliothis zea (Lepidoptera: Noctuidae) and their central projections

The thoracic mechanoreceptors of the wings and their central projections in the noctuid moth Heliothis zea (Lepidoptera: Noctuidae) were investigated using cobalt chloride infiltration methods. The different mechanoreceptors, tegula, campaniform sensilla, and chordotonal organ were identified as being present in the wing bases. The forewing and hindwing bases were innervated by two large nerve trunks (IIN1 and IIIN1, respectively). Terminal projections for both wing bases included massive regions within the fused meso-metathoracic and prothoracic ganglia, as well as direct projections to the suboesophageal ganglia. The terminal fields of IIIN1 were exclusively ipsilateral, whereas those of IIN1 also were contralateral. The relationship of these sensory mechanoreceptors to the neural basis of evasive flight behaviours is discussed.     

Hox and wings

In many bilaterian phyla, appendages are morphological traits that characterise the identity of the various body parts. In pterygote insects, wings are dorsal appendages on the thorax. The famous "bithorax" fly created by Ed Lewis is the emblematic example of the role of Hox genes.1 Now, Tomoyasu et al.,2 using classical genetics, transgenesis and RNAi, have examined the function of thoracic Hox genes in the beetle Tribolium castaneum. Beetles have rigid elytra in place of the first pair of wings. Instead of the expected transformation of the elytron into a wing, loss of Hox genes' function leads to the homeotic transformation of the second pair of dorsal appendages, the wings, into elytra. This has important consequences for the way that we see the role of Hox genes in development and evolution.     

Variations in the arrangement of sensory bristles along butterfly wing margins

The surfaces of insect wings exhibit numerous sensilla, which have been suggested to have a behavioral function. Some evidence suggests that the sensory bristles along the wing margin of lepidopteran insects (butterflies and moths) are involved in the regulation of wing movement. We investigated the arrangement of sensory bristles along the wing margins of 62 species of papilionoid butterflies, using light-microscopic examination of mounts of whole wings after removing the scales surrounding the bristles. In the majority of the wings examined, bristles were located on the ventral wing surfaces and were continuously distributed along the wing margins, except in the vicinity of the wing bases. In some wings, bristles were also located on the dorsal wing surfaces, and were continuously or discontinuously distributed along the wing margins of different species. In a minority of the species studied, we observed bristle distribution in the vicinity of the wing base, discontinuous bristle distribution on both the dorsal and ventral wing surfaces, or an absence of bristles along the wing margins. This variation in the arrangement of bristles along the wing margins is discussed in relation to the reception and transmission of sensory information in the wings.     

Double rotation of the opening (closing) elytra in beetles (Coleoptera)

Transient movements of the elytra (opening and closing) were filmed in beetles tethered from below. A total of 39 specimens of 18 species representing 11 families were examined. Bright markers glued to the elytra were traced frame by frame. Body-fixed 3D traces of apical and shoulder markers were reconstructed. Shapes of traces reflected different steps of elytral movement and different types of flight. Flat circular arcs were fitted to scattered traces using the least square method. The rotation axis of the apical marker was always directed at the contralateral side. The trace of the shoulder marker was, as a rule, non-parallel to the apical trace. Usually, the shoulder marker on the costal edge of the elytron uniformly supinated in the course of adduction of the apical marker. Traces of opening and closing coincided, hence the double rotation of the elytron had one degree of freedom. The elytron to body articulation in beetles is, presumably, a spherical mechanism with two separate but linked drives for a broad swing during opening (closing).     

Significance of hind wing morphology in distinguishing genera and species of cantharid beetles with a geometric morphometric analysis

There remain some difficulties in delimitation of related genera or sibling species for cantharid beetles, because the traditionally taxonomic method and morphological characters have not been updated or introduced. In the present study, we firstly use the landmark-based geometric morphometrics to analyze and compare the hind wings of nine species belonging to three genera of Cantharinae to ascertain whether this approach may be used as a reliable method in the study of the taxonomy of this group. The results show that the shape differences of the hind wings among genera seem more variable than that within each genus, and the variations for each species are different from one another, as shown in the principal component analyses. And the canonical variates analyses show that there are significant differences among the genera and the species of each genus, which demonstrates that the hind wing shape can be diagnostic for both generic and specific identification of the cantharid beetles. This study sheds new light into clarifying the taxonomic uncertainties of Cantharidae, and lays a foundation for further studies on the evolution of the cantharid hind wing shape.     

Wing folding and the functional morphology of the wing base in Coleoptera

The wing unfolding of Pachnoda marginata was examined using digital video (50 half-fps) and high speed video sequences (1000 fps), and the skeleto-muscular apparatus of the metathorax was described. Left and right hind wing are able to promote independently of each other. The hind wings do not unfold instantly when the elytra are lifted and may also reach the flight position (and beat) while still folded. Wing promotion is exhaustible and the time needed for unfolding varies considerably. These observations strongly suggest a muscular control. Wing unfolding is probably triggered by contraction of M. pleura alaris and a resulting proximad movement of the 3rd axillary sclerite, pulling the Media posterior backwards, while the Radius anterior is held by the basalar muscle as the antagonist. Our findings are in clear contrast to the earlier assumption that the hind wings of Coleoptera either unfold or fold due to intrinsic elasticity. The specific wing folding and unfolding mechanisms are autapomorphic character states of Coleoptera. They were maintained during evolution even though considerable variations of skeletal thoracic structures, musculature and venation occurred. (Additional material is available from the Zoology web page: http://www.urbanfischer.de/journals/zoology).     

Evolutionary patterns of hind wing morphology in dung beetles (Coleoptera: Scarabaeinae)

The shape of wings can be a good predictor of adaptations to different selective pressures and the value of wing features in taxonomy and phylogeny has long been recognized. In our investigation of the hind wing evolution of dung beetles (Scarabaeinae) we use geometric morphometrics combined with a cladistic approach. The variations of entire hind wings and of three specific regions of 80 dung beetle species were investigated using 19 landmarks and outline data. Extensive evidence indicates that the wing as a whole and the three separate regions were under different selective pressures. The detailed evolutionary patterns of the three regions and the reconstruction of the ancestral forms were computed by mapping the geometric morphometrics data onto a tree based on a cladistic character analysis.     

An artificial non-flying mutation to improve the efficiency of the ladybird Harmonia axyridis in biological control of aphids

The use of coccinellids in the biological control of aphids is restricted to the release of larvae because adults tend to fly away. Non-flying adults may stay longer in one place and so they and their progeny could give longer term protection to plants. This work is an attempt to produce a non-flying population by the use of a chemical mutagen and selection of adults with wing malformations through their subsequent generations. These adults are characterized by open elytra and extended wings. Some general features of this mutation were disclosed. The mutation is either unexpressed or results in malformed wings. It also seems recessive and lethal when homozygous. The adults with the mutation suffered a high level of mortality and a drastic reduction in reproductive capacity that prevents their mass rearing for biological control. This study revealed a negative relationship between wing malformations and reproductive capacity. Nevertheless, when adults with the mutation were released in greenhouses containing cucumbers infested with the aphid Aphis gossypii, they remained on the plants in higher numbers and laid eggs over a longer period of time than the control adults but their progeny were less numerous.