Mosquitoes exposed to DEET once are less repelled by it a few hours later, study claims

Mosquitoes are able to ignore the smell of the insect repellent DEET within a few hours of being exposed to it, according to research published February 20 in the open access journal PLOS ONE by James Logan, Nina Stanczyk and colleagues from the London School of Hygiene & Tropical Medicine, UK.

Though most insects are strongly repelled by the smell of DEET, previous studies by Logan's research group have shown that some flies and mosquitoes carry a genetic change in their odor receptors that makes them insensitive to this smell. The new results reported in the PLOS ONE study uncover a response in mosquitoes based on short-term changes, not genetic ones.

Follow the odor and CO2: Flight patterns reveal how mosquitoes find hosts to transmit deadly diseases

The carbon dioxide we exhale and the odors our skins emanate serve as crucial cues to female mosquitoes on the hunt for human hosts to bite and spread diseases such as malaria, dengue and yellow fever.

Two entomologists at the University of California, Riverside have now performed experiments to study how female Aedes aegypti -- mosquitoes that transmit yellow fever and dengue -- respond to plumes of carbon dioxide and human odor.
The researchers report in the Oct. 15 issue of the Journal of Experimental Biology that puffs of exhaled carbon dioxide first attract these mosquitoes, which then proceed to follow a broad skin odor plume, eventually landing on a human host.
The results from the study by Ring Cardé, a distinguished professor of entomology at the University of California, Riverside, and Teun Dekker, formerly a graduate student in Cardé's lab and now an assistant professor at the Swedish University of Agricultural Research, could clue scientists on how odors can be used in traps for intercepting and capturing host-seeking mosquitoes.

Adults

Adult mosquitoes are usually 3-6 mm long, but some toxorhynchitines, which are the largest mosquitoes, are nearly 20 mm long. As in other insects the body has three principal regions, the head, thorax and abdomen. The head is the anterior section of the body bearing the compound eyes, antennae and mouthparts. It is separated from the second or intermediate division of the body, the thorax, by a membranous tube called the cervix, or neck. The thorax is comprised of three segments, each bearing a pair of legs. The largest or middle segment of the thorax bears the wings and the reduced third or posterior segment bears the halteres. The abdomen is the third, posterior division of the body. It consists of ten segments and the terminal genital structures involved in reproduction. The male genitalia are extremely important in species identification in most genera. Except for sclerotized elements of the alimentary canal and the reproductive system, the internal anatomy is not used in mosquito taxonomy. The presence or absence of setae and the placement and color of scales provide important characters for species identification.

Mosquitoes are much more extensively clothed with scales than other Diptera which have scales. The scales are either pale or dark. Dark-colored scales vary from reddish brown to black depending on the species and to a lesser extent on the age of the specimen. Some mosquitoes have metallic-colored scales which appear blue, green, gold, red or violet in certain light. Pale-colored scales are white, silver, yellow or gold. Scales may be broad and flat, narrow and curved, or erect and forked apically; those comprising the wing fringe are fusiform in shape. Scales are easily rubbed from the body; hence, important taxonomic characters may be missing in rubbed specimens. The color of the integument also varies interspecifically, and this may influence the overall color of the scales.

Abdomen Adult Mosquito

The abdomen consists of 10 segments of which only eight are distinctly visible. Segments IX and X are reduced and more or less modified as part of the external structures involved in reproduction, the genitalia. In some species segment VIII is sexually differentiated from the preceding segments, and in such cases it is also treated as part of the genitalia. Each segment consists of a dorsal sclerite, the tergum (pl. terga), which is joined by an elastic pleural membrane to a ventral sclerite, the sternum (pl. sterna). Intersegmental membrane connects the terga and sterna of adjacent segments. It is the elasticity of the pleural and intersegmental membranes that allows the abdomen of the female mosquito to become distended when she is taking a blood meal or when she is gravid and full of eggs. The terga are collectively referred to as the dorsum; likewise the sterna as the venter. Both the dorsum and venter are clothed with scales in culicines and toxorhynchitines, but are usually bare or have few scales in anophelines. The terga may have basal, apical or lateral patches of pale scales which provide useful taxonomic characters. Scale patterns are generally not so evident on the sterna.

The male genitalia of mosquitoes (segments VIII-X) undergo a clockwise or counter clockwise rotation of 180° shortly after emergence which results in a reversal of the dorsal and ventral structures. To avoid confusion, these structures are referred to in their original positions before rotation (dorsal and ventral, or tergal and sternal) even thought they appear to occupy the opposite positions in the mature specimen. Modifications of the genital parts are more or less complex and varied according to the genera and species, thus providing important taxonomic characters. Female genitalia have been largely neglected in mosquito taxonomy, but certain prominent structures have been used to a limited degree in recognising species of certain genera.

Head Adult Mosquito

Mosquitoes, both males and females, can be distinguished from all other Diptera by their long, slender, scaled proboscis. A few other flies also have an elongate proboscis, but it is not covered with scales.

Males of most mosquitoes can be distinguished from females by their verticillate (bushy, not plumose) antennae (sing. antenna). The sections, flagellomeres, of the third antennal segment, the flagellum, bear whorls of setae in both sexes, but in females these setae are relatively short. In the males of most species (not all), the setae on all but the last flagellomere are several times longer and more numerous than in the females.

The head is globular and largely comprised of a pair of prominent compound eyes. The eyes may be touching or separated both dorsally and ventrally. The dorsal surface of the head behind the compound eyes consists of the indistinguishable vertex (anteriorly) and occiput (posteriorly), and is more or less covered by erect and/or decumbent scales. The erect scales are usually truncate or forked apically and often differ in colour from the decumbent scales. They may be numerous, occurring on the vertex and the occiput, or they may be restricted to the occiput, usually in a transverse row. The decumbent scales may be narrow and curved or broad and flat. A row of anteriorly directed, ocular setae arise along the margin of each eye. One or more pairs of interocular setae located at the junction of the eyes (interocular space) are usually longer and project more strongly cephalad than the other ocular setae. The narrow anterior part of the vertex between the eyes bears the frontal tuft characteristic of most Anopheles species. The tuft composed of long setiform scales curving forwards over the frons and clypeus, together with setae and slender but shorter scales.

The antennae, maxillary palpi (sing. maxillary palpus) and the proboscis arise from a frontal area more or less transcribed by the compound eyes. Each antenna is divided into three segments. The basal segment, the scape, is hidden behind the greatly enlarged second segment, the pedicel. The pedicel often bears a group of minute setae or a small patch of scales on its mesal surface. The third segment, the flagellum, is divided into 13 or 14 false segments called flagellomeres. The first flagellomere usually also bears a mesal patch of scales. The remaining flagellomeres are usually without scales. In females, the flagellomeres are about equal in size, but in males of most species the apical two flagellomeres are longer. In genera of tribe Sabethini (e.g. Wyeomyia and Sabethes) and certain other genera (e.g. Hodgesia) the antennae of males resemble those of the females in not having whorls of long setae.

The maxillary palpi are composed primitively of five false segments or sections called palpomeres. In both sexes of anophelines, and in the males of nearly all other genera, the palpus is elongate and all five palpomeres are distinct. A small lobe (palpifer) at the base of the maxillary palpus gives the palpus of males a six-segmented appearance. Because of reduction of the apical palpomeres, the palpus of females appears to be composed of two or three palpomeres. The basal half of the first palpomere in both sexes lacks scales and setae. The rest of the palpus is covered with scales, and the three distal palpomeres (3-5) of males usually have setae.

The proboscis is covered with appressed scales and terminates in a pair of lobe-like labella (sing. labellum). The visible part of the proboscis is largely composed of the labium, which encircles the piercing stylets of the mouthparts. The proboscis is usually more or less straight or slightly curved, but in Toxorhynchites it is bent strongly backwards (reflexed). The labium is closed dorsally by the labrum. The distal part of the proboscis is distinctly swollen in some mosquitoes (e.g. Malaya and males of Ficalbia and Mimomyia).

Thorax Adult Mosquito

The thorax is comprised of three segments. Most of the thorax belongs to the second or middle segment, the mesothorax. The prothorax and metathorax are much reduced, especially dorsally. Each segment bears a pair of legs, while the mesothorax carries the wings and the metathorax bears the knob-like halteres (sing. halter). The dorsum of the prothorax is represented by laterally displaced halves, which are divided transversely into a lobe-like antepronotum and a flattened postpronotum. Dorsally the mesothorax occupies almost the entire thorax in the form of the large sclerite, the scutum, which is covered with scales and usually bears rows of setae. Those along the midline are called the acrostichal setae while the rows on either side are referred to as the dorsocentral setae. The arrangement of the scales and the patterns they form on the scutum are important in species identification. A small oblong or triangular sclerite called the paratergite lies at the edge of the scutum between the mesothoracic spiracle and the base of the wing. The presence or absence of scales and setae on the paratergite are often important in generic and species identification. Behind the scutum is the crescent-shaped scutellum. The scutellum is usually tri-lobed (except in Anopheles, Bironella and Toxorhynchites), covered with scales and bears groups of setae along the posterior edge. A dome-shaped mesopostnotum lies below and behind the scutellum. This area is usually bare, but scales and/or setae are present in some species and taxa. The dorsum of the metathorax is represented by an inconspicuous sclerite behind the mesopostnotum.

The lateral side of each thoracic segment is known as a pleuron (pl. pleura). In winged insects, a vertical ridge divides the pleuron into an anterior episternum and a posterior epimeron, and each of these may be divided longitudinally into a dorsal anepisternum and a ventral katepisternum. In mosquitoes, only the mesopleuron has the structure of a more generalised thoracic segment. The propleuron is represented by the partially fused, V-shaped proepisterna (sing. proepisternum) located between the forecoxae and the neck (cervix). Scales and setae borne on the proepisterna are often of taxonomic importance. The upper proepisternal scales and upper proepisternal setae are located above the base of each forecoxa while the lower proepisternal scales and lower proepisternal setae are borne in a mesal position below the neck and between the coxae. The upper and lower patches of scales may be contiguous. The anteprocoxal membrane situated between the forecoxa and the proepisternum and the postprocoxal membrane borne between the forecoxa and the mesothorax may also bear scales.

The mesopleuron is divided into unequal anterior and posterior sclerites by a mesopleural suture. The larger anterior sclerite, the mesepisternum is subdivided by a longitudinal anapleural suture into a dorsal mesanepisternum and a ventral mesokatepisternum. The anepisternum is further divided by a diagonal anepisternal cleft into anterior and posterior sections. The anapleural suture is indistinct in most mosquitoes, except Uranotaenia, so that the posterior section of the anepisternum (prealar area), which bears the prealar setae, appears to be a dorsal extension of the mesokatepisternum. The anterior portion of the anepisternum bears the mesothoracic spiracle. An important group of setae, the prespiracular setae, arise from a small sclerite (prespiracular area) on the anterior side of the spiracle. These setae are usually small and inconspicuous and arise immediately posterior to the postpronotal setae, which can be mistaken for the prespiracular setae. The presence or absence of prespiracular setae in combination with other characters is important in distinguishing mosquito genera. The remainder of the anterior section of the anepisternum is arbitrarily divided into three areas, the hypostigmal area, subspiracular area; and postspiracular area, which may bear taxonomically important setae and/or scales. The mesokatepisternum is well developed and bears two important groups of setae, the upper mesokatepisternal setae and lower mesokatepisternal setae. Scales are also associated with these groups of setae, termed the upper mesokatepisternal scales and lower mesokatepisternal scales by association. The mesepimeron, a rectangular sclerite located behind the mesopleural suture, is also divided transversely. The dorsal mesanepimeron, or simply the mesepimeron, is quite large and bears patches of setae and scales of taxonomic importance, while the ventral mesokatepimeron is an insignificant strip of bare cuticle. A small triangular sclerite known as the mesomeron is located below the mesepimeron and between the mid- and hindcoxae. The positional relationship of the mesomeron to the base of the hindcoxa is important in the higher classification and generic recognition of mosquitoes.

The metathorax has limited value in mosquito taxonomy. It is greatly reduced and only the metapleuron and the metameron are readily distinguishable. Metepisternal scales are present in Toxorhynchites and a few sabethine genera (Topomyia and some species of Malaya and Tripteroides). The metameron sometimes bears a patch of scales.

Wings Adult Mosquito

The wings of mosquitoes are long and slender, and the wing veins are covered with scales. The terminology currently in use for the veins is adopted from Colless & McAlpine (1970). The principal longitudinal veins of mosquitoes (from anterior to posterior) are the costa, subcosta, radius, media, cubitus and anal vein. The branches of these veins are indicated by numbers, thus the anterior branch of the media is media-one (vein M1) and the posterior branch is media-two (vein M2). Short, usually transverse veins between two longitudinal veins are known as crossveins. In mosquitoes, the usual crossveins are the humeral crossvein between the costa and subcosta, the radiomedial crossvein between the radius and media, and the mediocubital crossvein between the media and cubitus. Areas of wing membrane delimited by veins or by veins and the wing margin are termed cells. A cell is named after the vein immediately anterior to it or after the posterior element if the vein is formed by the fusion of two vein branches, for example, cell R5 is posterior to radius-four-plus-five (vein R4+5). The cell membranes are covered with minute spicules called microtrichia (sing. microtrichium). They are especially minute in species of Uranotaenia.

The veins are clothed with scales on both the dorsal and ventral surfaces, and the apex and posterior margin of the wing bear a fringe of scales wing fringe. The scales vary considerably in size and shape on different parts of the wing. The scales are usually dark, but many species have some pale scales either intermixed or in patches. Distinct patches of pale and dark scales known as wing spots occur in most Anopheles species and species of Aedeomyia, Finlaya, Orthopodomyia, the Mimeticus Group of Culex (Culex), Orthopodomyia, Psorophora and Uranotaenia, and these play a major role in species identification. A standardised nomenclature for the wing spots of these mosquitoes was developed by Wilkerson & Peyton (1990). The alula, a small lobe located posteriorly at the base of the wing, usually bears a line of marginal or submarginal scales of different shapes or posture in various taxa, and the upper calypter, one of two lobes connecting the alula to the thorax, sometimes bears setae or hair-like scales on its margin.