Susan B. McIver

Fine structure of antennal sensilla coeloconica of culicine mosquitoes

Abstract The sensilla coeloconica (pegs in pits) previously mis-identified as campaniform organs, at the tip of the antennae of female Aedes aegypti L. and Culexpipiens (L.) are described. Each sensillum is innervated by three bipolar neurons: the dendrites of two are unbranched whereas the distal portion of the third is folded into tightly packed lamellae. One unbranched dendrite extends to the tip of the peg and the other ends near the base of the peg. The lamellae-bearing dendrite terminates 4-5 μ beneath the base of the peg. Chemo- and thermoreception are the proposed functions for the sensillum.

Fine structure of antennal grooved pegs of the mosquito, Aedes aegypti

SummaryThe grooved pegs also referred to in the literature as type A3 setae, thornshaped hairs, pegs, and sensilla basiconica of female Aedes aegypti (L.) are described. Externally the sensillum appears as a short, thick-walled, non-articulated peg with usually 12 grooves in the wall and bearing one terminal pore. Usually three, occasionally four or five, neurons with unbranched dendrites innervate each peg. The dendrites are encased in a prominent cuticular sheath from the ciliary region to the tip of the peg. Three sheath cells are associated with each sensillum.Although the grooved pegs have the structure commonly associated with contact chemoreceptors they function exclusively in the reception of air-borne stimuli.

Fine structure of antennal putative thermo‐/hygrosensilla of adult Rhodnius prolixus Stål (Hemiptera: Reduviidae)

At least five nonporous sensilla with inflexible sockets (npsensilla) occur on each antenna of both sexes of adult Rhodnius prolixus. Externally the sensillum appears as a short, rounded peg set into a pit surrounded by a depression. A very electron‐dense material occurs in the peg lumen and the inner aspect of the pit. Filamentous extensions of this material radiate into the overlying outlets.

Structure of sensilla trichodea of female Aedes aegypti with comments on innervation of antennal sensilla

Abstract On each antenna of female Aedes aegypti four types of sensilla trichodea are distinguishable: long and short, pointed-tipped ones and blunt-tipped types I and II. All types are innervated by two neurones, except the short, pointed-tipped trichodea with which only one neurone is associated. Both pointed-tipped types have unbranched dendrites and relatively thicker hair walls perforated by fewer pores than the blunt-tipped types. The long, pointed-tipped trichodea are 50–60 μm in length and the short ones 20 μm. In both blunt-tipped types the dendrites divide and the hair walls are perforated by numerous pores. Blunt-tipped type I trichodea are 20–40 μm in length and taper somewhat whereas the type II hairs are 11–13 μm in length, do not taper appreciably, and have exceedingly thin walls. All types of sensilla trichodea are olfactory receptors and the blunt-tipped type I are known to respond to repellents. An investigation of the possibility of axon fusion in the flagellar nerve gave negative results. Each flagellar nerve is composed of an estimated 2058 neurones. A majority of these, namely 93%, are involved in the recognition and discrimination of olfactory cues. Approximately 65% of the neurones respond to repellents, 5% to mechanical stimuli, and 2% to heat transferred by convection.

Fine structure of the antennal tip of the crabhole mosquito, Deinocerites cancer Theobald (Diptera : Culicidae)

Abstract The antennal tips of both sexes of Deinocerites cancer Theobald bear sensilla coeloconica (pegs in pits) and internal sensilla with bipolar neurons that lack any connection with the cuticle. Each sensillum coeloconicum is innervated by 3 neurons. The dendrites of 2 neurons extend unbranched into the peg, whereas that of the third terminates below the peg base and is divided into numerous lamellae and microvilli. In males, the lamellated dendrite is much larger and more complex in organization than in females, a feature which is probably related to the unusual pupal finding and attendance phases of the mating behavior of the male. The 2 neurons with unbranched dendrites probably respond to heat transferred by convection. Possibly, the lamellated dendrite may be sensitive to infrared radiation or mechanical stimuli. Each internal sensillum in male D. cancer has one bipolar neuron with a dendrite divided into lamellae and microvilli. Females have similar internal sensilla plus a type with 2 bipolar neurons, the dendrite of one is unbranched and that of the other is lamellate in structure. The internal sensilla represent an intermediate form between the usually accepted Type I and Type II arthropod sensilla. Based on similarity in fine structure it is suggested that the internal sensilla have the same proposed functions as the sensilla coeloconica.

Fine structure of antennal sensilla of larval Toxorhynchites brevipalpis theobald (Diptera : Culicidae)

Abstract The present investigation of the fine structure of antennal sensilla of fourth instar Toxorhynchites brevipalpis was undertaken to aid in our understanding of the sensory basis of behavior of predacious mosquito larvae and make contributions to the limited knowledge of the sensilla of immature, aquatic insects. Each antenna has 26 neurons which innervate 5 types of sensilla. At the antennal tip are a cone sensillum with a protuberance, a peg sensillum, and an unbranched hair sensillum. Two other unbranched hair sensilla, a branched hair sensillum, and a campaniform sensillum occur in the midregion. Another campaniform sensillum is at the base. Each cone sensillum is innervated by 11 neurons which occur in 6 distinct groups called neuronal units, each consisting of 1–3 neurons with inner and outer sheath cells. Medially on the cone is a small protuberance with a neuronal unit containing 2 neurons. The dendrite of one neuron extends into the protuberance where distally it branches somewhat, while that of the second terminates below the base of the cone and divides into lamellae. Each peg sensillum is innervated by 4 neurons. The dendrites of 3 extend to the peg tip, while that of the fourth ends subterminally. A role in chemoreception is suggested for all the neurons associated with the cone, protuberance, and peg, except for the lamellated dendrite, whose function is obscure. Each branched hair and campaniform sensillum is innervated by one neuron which terminates as a tubular body. Two neurons are associated with each unbranched hair sensillum. The dendrite of one terminates at the hair base as a tubular body, while that of the second extends into the hair shaft. In the proximal region of the hair the latter dendrite contains many microtubules in an electron-dense material, a feature reminiscent of tubular bodies. All neurons associated with the hair and campaniform sensilla probably function in mechanoreception. These results are generally similar to those reported for Aedes aegypti larvae, which are primarily filter-feeders. Apparently any differences in sensory requirements between predatory and filter-feeding larvae are not reflected in the number or structure of antennal sensilla.

Fine structure of antennal sensilla of male Aedes aegypti (L.)

Abstract The terminal two antennal segments of male Aedes aegypti bear the same variety of sensillar types as the females antenna, namely, sensilla chaetica, sensilla coeloconica, sensilla ampullacea, grooved pegs, and four types of sensilla trichodea: long and short, pointed-tipped trichodea and blunt-tipped types I and II. Each type of sensillum has a similar fine structure in both sexes. Of the 514 neurones which innervate these sensilla in the male, 91% are olfactory receptors, 7% mechanoreceptors, and 2% thermoreceptors. The total number of neurones in the male is about four times fewer than in the female, but the ratio of those responding to the various stimuli is similar. The sensilla studied herein probably mediate stimuli involved in location of suitable resting sites and nectar meals. In addition they are apparently involved in location of vertebrates as recent studies indicate that certain male mosquitoes are attracted to hosts to bring them into the proximity of the females for mating. This host finding behaviour of males would explain why they have the same sprectrum of sensillar types as do females.

Experiments on biting and gorging behaviour in the black fly, Simulium venustum

ABSTRACT. Factors which initiate ‘biting’ (i.e. probing, piercing and tasting collectively) and gorging responses, were studied in the black fly, S. venustum using an artificial feeding method. A positive relationship was found between biting activity and the magnitude of the temperature differential between the feeding surface and the air above it. Although many investigators consider temperature to be a probing stimulus, it is argued that an equally consistent interpretation could regard temperature as only a very short range host‐location cue. The stimulus to probe could be contact with the feeding substrate. Of the compounds tested as gorging stimulants ATP and ADP proved most potent, followed by AMP and adenosine, followed by cAMP. The compounds GTP, CTP and UTP were all on the borderline of statistical significance as gorging stimulants. It is suggested that the host‐location phase, including the biting responses, represents appetitive behaviour leading to the consummatory response of repetitive pumping (gorging) stimulated by ATP, etc.

Fine structure of tarsal sensilla of Aedes aegypti (L.) (Diptera: Culicidae)†

The tarsi of all three pairs of legs of both sexes of Aedes aegypti (L.) bear spine sensilla, five types of hair sensilla, which are designated A, B, C1, C2 and C3, and campaniform sensilla. Type A and B hairs, spines, and cam‐paniform sensilla are innervated by one neuron with a tubular body, a characteristic of cuticular mechanoreceptors. In particular the hairs and spines are tactile receptors and the campaniform sensilla are proprioceptors. The C1, C2, and C3 hair sensilla have the morphological features of contact chemoreceptors. Type C1 and C3 hairs are innervated by five and four neurons, respectively, which extend to the tip of the hair. Type C2 is innervated by five neurons, one of which terminates at the base of the hair in a tubular body while the remaining four extend to the tip of the hair. The role of the type C hairs in oviposition behavior, nectar feeding, and recognition of conspecific females is discussed. Presumed efferent neurosecretory fibers occur near the spine and hair sensilla.

Palpal sensilla of selected anopheline mosquitoes.

Palps of both sexes of Anopheles stephensi, A. albimanus, A. quadrimaculatus, and female A. gambiae were examined and found to be equipped to perceive both mechanical and olfactory stimuli. Present on the palps are sensilla chaetica, tactile and/or air current receptors, campaniform sensilla, proprioceptors, and thin-walled capitate pegs, olfactory receptors probably sensitive to carbon dioxide. The fine structure of each type of sensillum is described. Mosquitoes have various types of antennal and palpal sensilla which are responsible for the reception of external stimuli, including those involved in host finding and selection. Several investigators have reported on the detailed structure (Elizarov and Chaika, 1972; McIver, 1971, 1972; McIver and Charlton, 1970; McIver and Hudson, 1972) and function (Bassler, 1958; Kellogg, 1970) of palpal sensilla of culicine mosquitoes, but information about those on anopheline mosquitoes has been lacking. This work was conducted to (a) determine the types of palpal sensilla on both sexes of Anopheles stephensi Liston, Anopheles albimanus Wiedemann, and Anopheles quadrimaculatus Say, and female Anopheles gambiae Giles and (b) describe the fine structure of the sensilla on female A. stephensi. MATERIALS AND METHODS Specimens of A. stephensi, A. gambiae, A. albimanus, and A. quadrimaculatus were obtained from laboratory colonies maintained, respectively, at the Department of Parasitology, Department of Botany, University of Toronto, Central American Malaria Research Station, San Salvador, and the Center for Disease Control, Atlanta, Georgia. For transmission electron microscopy, palps were removed from mosquitoes immersed in Karnovskys fixative (Karnovsky, 1965) at pH 7 and 4 C and left in the fixative for 18 hr. After rinsing in 0.05 M Sorensens 10% sucrose buffer, pH 7, the tissue was postfixed in 1% Os04 in veronal acetate buffer at pH 5 and at room temperature for 2 hr. Dehydration through ethanol was followed by embedding in Spurrs low viscosity epoxy medium (Spurr, 1969). For scanning electron microscopy specimens were fixed in 5% formalin and dehydrated through a graded series of ethanols to xylene (Slifer, 1972), attached to the stubs with silver conductive paint, Received for publication 9 December 1974. coated with gold during spin rotation, and examined in a Cambridge Stereoscan microscope. For counting the sensilla, double coverslip mounts of heads were prepared as described by McIver (1969). All numerical results are means of at least 10 counts or measurements. Statistical significance was determined using Students t test at the 5% level and adjustment for multiple comparison effect was made. Permeable areas in the cuticle were demonstrated by staining whole specimens with crystal violet (Slifer, 1960) from 30 min to 2 hr.