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Many of the generalizations made about mosquito behavior and physiology are based on the extensive research on the yellow fever mosquito, Aedes aegypti (L.). However, sufficient differences exist among the numerous species of mosquitoes that make many of these generalizations unwarranted. Some of the specific differences between Ae. aegypti and other mosquito species, particularly the important malaria vector Anopheles gambiae, are discussed.

The cuticular hydrocarbons of female Anopheles gambiae Giles sensu stricto and Aedes aegypti (L.) mosquitoes were analyzed before and after they mated. In An. gambiae, the proportions of the two cuticular hydrocarbon components, n-heneicosane and n-tricosane, were significantly reduced as the female aged and after it mated. There were no changes in the hydrocarbon composition of males after they mated. Hydrocarbon extracts from mated and unmated An. gambiae females as well as those from males caused a reduction in the rates of female insemination when they were applied to unmated females. Female Ae. aegypti showed significant changes in the proportions of n-heptadecane, n-pentacosane and n-hexacosane in their cuticles after mating. These data suggest that cuticular hydrocarbons may play some role in chemical communication during mosquito courtship.

The survival, metabolic reserves, and host-seeking response of unfed Anopheles gambiae Giles sensu stricto females of different size classes were investigated from 1 to 6 d after emergence. In addition, the effect of blood meal size and frequency of feeding on the accumulation of metabolic reserves and reproductive development were investigated during the 1st gonotrophic cycle. Large females survived longer and contained significantly more protein, glycogen, and lipid at emergence than did small females. Over 6 d, the host-seeking response of large mosquitoes was greater than that of small ones. There was no gonotrophic development in unfed small females after emergence, whereas < or = 52% of large females developed oocytes to Christophers stage II. One blood meal, given by enema, resulted in gonotrophic development to stage II in small females, whereas only large females matured oocytes to Christophers stage V. In both large and small females, egg development was related to the size of the blood meal, but large females were better able to utilize the meals. Two successive blood meals enhanced the ability of small females to develop eggs. The pregravid condition commonly observed in An. gambiae appears to be determined by insufficient metabolic reserves at emergence, and the probability for small individuals to be pregravid is greater than for large mosquitoes because of the need to build up sufficient energy reserves before gonotrophic development can occur. The consequence of this behavior in relation to malaria transmission is discussed.

Blood ingestion by the female mosquito initiates a series of endocrine events that is dominated by juvenile hormone, ecdysteroids, and several peptide hormones, resulting in the maturation of a batch of eggs. The mechanisms of hormone release and their roles during the previtellogenic, vitellogenic, and postvitellogenic phases are discussed. Arch. Insect Biochem. Physiol. 35:491–512, 1997. © 1997 Wiley-Liss, Inc.

Mechanisms initiating autogenous egg development were studied using a selected strain of Asian tiger mosquito, Aedes albopictus (Skuse), that required a sugar meal to develop eggs autogenously. Caloric intake and the abdominal distention produced by ingesting sucrose solutions were interrelated in their effects on autogeny. Distention of the abdomen with 2 microliters of saline, with no caloric intake, induced autogenous egg maturation in 66% of the females. Abdominal distention produced by 2 microliters of saline did not induce egg development if the ventral nerve cord was transected. However, eggs were produced when females ingested 200 micrograms of sucrose in 2 microliters of water following ventral nerve cord transection. A meal containing at least 100 micrograms of sucrose was required for egg development if abdominal distention was1 microliter. Mating influenced autogeny in only 10% of the population. Neither distention, caloric intake nor mating affected the number of eggs that matured.

When the terminal abdominal ganglion (TAG) of female Aedes aegypti was surgically removed and the mosquitoes blood-fed on a rat, after 2 days neither uric acid nor hematin were excreted and water loss was significantly reduced. Urate was removed from hemolymph and accumulated in the midgut. TAG removal also prevented the elimination of a dye dissolved in sugar. Similar results were obtained with mosquitoes in which defecation was mechanically prevented by sealing the anus. Decapitation or transection of the ventral nerve cord anterior to the second abdominal ganglion did not inhibit defecation. Egg development was not affected by TAG removal. Reimplantation of a denervated TAG partially restored uric acid and water excretion. These results suggest that the TAG specifically controls defecation, in part humorally, but it does not appear to interfere with the function of the Malpighian tubules.

It is essential for cells to know where they are and what the environment around them is like. Single-celled organisms may have to detect changes in nutrients, temperature, mechanical pressure, electromagnetic fields, light, and the metabolic products of other cells of the same or different species, and make appropriate responses. This process becomes more critical for the cells of multicellular organisms, because beginning a few moments after the fertilized egg cell first divides, the individual cells must begin communicating with each other to coordinate their activities. As development proceeds, cells not only need to respond to changing conditions but also to exchange information that will determine their specialized identity and the positions they will assume in the mature organism. The information from the outside of the cell must be translated and converted to specific cellular responses in a series of steps referred to as signal transduction pathways.

Publisher Summary This chapter describes the excretory system in insects. Insects are able to occupy a wide variety of terrestrial niches and effectively deal with issues of water balance largely because of two important adaptations: their impermeable exoskeleton and an excretory system of considerable sophistication. The excretory system maintains the internal environment of the insect by separating and eliminating metabolic wastes and other toxic substances from body compartments. Because these wastes are often dissolved in water, excretory processes are also closely associated with osmoregulation and the maintenance of water balance. The Malpighian tubules are the primary excretory organs of insects and the driving force for excretion through them is the movement of ions across the epithelium. Superimposed on the excretion of primary urine by Malpighian tubules is a second system involving the rectum of the hindgut that recovers most of the ions and water, adjusting the excretory product so that it achieves the necessary osmoregulatory balance for the insect and excretes the secondary urine. In insects, the need for water conservation may have been the driving force for the incorporation of their nitrogen wastes into uric acid, which is an ideal excretory product for small terrestrial animals. Uric acid is highly insoluble in water and therefore fails to reach toxic levels in body fluids, so it requires about 50 times less water to dilute than does ammonia. Insects pay a high price for the benefits they derive from employing uric acid as a way to excrete nitrogen and still maintain a positive water balance. The synthesis of uric acid from protein results in the loss of several carbon atoms that could be used for other biosyntheses and requires a substantial amount of energy to build a larger, less toxic molecule.

Publisher Summary This chapter describes the structure and physiology of circulatory systems of insects. Although insects are also multicellular animals, they are small enough to allow diffusion to serve as a mechanism of metabolic exchange. The extensive tracheal system in insects carries oxygen to the cells, and, with the exception of the few species that supplement oxygen transport with hemoglobin, the circulatory system has little function in oxygen transport. The hemolymph is the major extracellular fluid in insects, and the major transport medium for the exchange of materials between cells, such as hormones, waste materials, and nutrients. Through its regulation of ionic and chemical composition, it maintains the proper internal environment for cells as an extracellular extension of intracellular fluids. The most important roles of insect hemolymph are to serve as a medium that bathes cells and transfers substances to and from them, as a reservoir of water and metabolic substances, as a medium for cellular and humoral defense, and, in soft-bodied insects, to provide the necessary hydrostatic pressure for molting and maintenance of body shape. It may also protect cells from freezing, contain substances that deter predation, and provide a medium for the retention of metabolic heat that allows the insect to remain active at low environmental temperatures. As a medium that is used primarily for the transport of chemical agents, its role is far less demanding than for vertebrate blood that has the absolute requirement for oxygen transport.

There are many striking similarities between the physiological systems of vertebrates and those of insects, but one conspicuous difference is in the way that oxygen is brought to the cells. This difference was unusual enough for Aristotle, in 350 BCE, to characterize insects as terrestrial animals that had no requirement to inhale. It was Malpighi, in 1669, who first identified the series of branching tubes, the tracheae (singular: trachea), that bring oxygen directly to tissues, a departure from the system in vertebrates, where air is regularly and often visibly drawn into the body to oxygenate body fluids that are then circulated to all cells.

Male accessory gland (MAG) substances of the mosquito, Aedes aegypti (L.), when transferred during mating or injected directly into females, significantly reduced the host-seeking behavior of the females when they were gravid. The effect on female behavior was modulated by the nutritional state of the male; compared to well-fed males, starved males contained less total protein in their MAGs, transferred less protein to females during mating, and had less of an effect on the inhibition of host-seeking behavior by the gravid females to which they mated. Juvenile hormone (JH) administered to starved males increased the levels of total MAG proteins in their accessory glands, but the host-seeking behavior of gravid females mated to JH-treated males was not significantly affected, suggesting that the production of specific substances did not increase. Injection of MAG homogenates from other mosquito species into Ae. aegypti indicated that there was some degree of specificity in the inhibition of the host-seeking behavior.

e may think of ourselves as being at the top of the food chain, but we have been nibbled upon by various bloodsucking insects since our primate ancestors first inhabited this planet. The bloodsuckers' swarming and biting behaviors are not only nuisances, but these insects also transmit many disease-producing parasites as they salivate into our bloodstreams. Malaria, the most infamous of these diseases, is estimated to cause more than two million human deaths each

Chikungunya virus (CHIKV) is an alphavirus transmitted by Aedes albopictus and Aedes aegypti mosquitoes in tropical areas of Africa, Asia, and the islands of the Indian Ocean. In 2007 and 2009, CHIKV was transmitted outside these tropical areas and caused geographically localized infections in people in Italy and France. To temporally and spatially characterize CHIKV infection of Ae. albopictus midguts, a comparison of viral distribution in mosquitoes infected per os or by enema was conducted. Ae. albopictus infected with CHIKV LR 5' green fluorescent protein (GFP) at a titer 10(6.95) tissue culture infective dose(50) (TCID(50))/mL, were collected and analyzed for virus dissemination by visualizing GFP expression and titration up to 14 days post inoculation (dpi). Additionally, midguts were dissected from the mosquitoes and imaged by fluorescence microscopy for comparison of midgut infection patterns between orally- and enema-infected mosquitoes. When virus was delivered via enema, the anterior midgut appeared more readily infected by 3 dpi, with increased GFP presentation observed in this same location of the midgut at 7 and 14 dpi when compared to orally-infected mosquitoes. This work demonstrates that enema delivery of virus is a viable technique for use of mosquito infection. Enema injection of mosquitoes may be an alternative to intrathoracic inoculation because the enema delivery more closely models natural infection and neither compromises midgut integrity nor involves a wound that can induce immune responses. Furthermore, unlike intrathoracic delivery, the enema does not bypass midgut barriers to infect tissues artificially in the hemocoel of the mosquito.

Most species of mosquitoes must engage in host-seeking behavior to find a vertebrate host and ingest a meal of blood in order to initiate and complete each oogenic cycle. Female Aedes aegypti fed to repletion almost immediately fail to respond to odor cues that would otherwise lead them to a host for another blood meal. This response is delayed for approx. 24h in females that ingest small blood meals. In both instances, during this period of inhibition, the hemolymph titer of Ae. aegypti Head Peptide I (Aea-HP-I; pGlu-Arg-Pro-Hyp-Ser-Leu-Lys-Thr-Arg-Phe-NH 2 ) rises to a peak, as measured with an Aea-HP-I radioimmunoassay (RIA). This homologous RIA is based on an antiserum that specifically binds Aea-HP-I (5–100 fmol range, 1:100,000 final dilution) as compared to other related peptides, Aea-HP-I and at least one other immunoreactive peptide were identified in pooled hemolymph from sugar-fed females with HPLC fractionation and the RIA. To determine whether Aea-HP-I affects host-seeking behavior, synthetic Aea-HP-I and related peptides were injected into non-oogenic females, which actively seek a host. Host-seeking behavior was inhibited by Aea-HP-I within a dosage range of 16.5–825 pmol/female, whereas related peptides differing by a few amino acids had no effect. The relatively high doses of Aea-HP-I, which elicited behavioral inhibition, were rapidly degraded in vivo as determined with the RIA of hemolymph from experimental females. Other associated behaviors were not altered by the Aea-HP-I injection, even at high doses. These results together suggest that Aea-HP-I regulates the observed behavioral inhibition in this mosquito species.

Aedes albopictus (Skuse) females were more likely to develop eggs after they ingested small blood meals than were Aedes aegypti (L.) when both species were maintained under the same suboptimal adult nutritional regimen. The longevity of adult female Ae. albopictus under conditions of starvation was also significantly greater than that of Ae. aegypti. Analyses of total body proteins, lipids, and glycogen indicated that the increased reproductive efficiency of Ae. albopictus may be a result of its greater reserves accumulated during a longer larval period.

Publisher Summary This chapter focuses on the spermathecas, a special pouch in the female in which spermatozoa are stored and maintained after mating. In more advanced insects, the male deposits seminal fluid or a spermatophore into the bursa copulatrix, and the sperm then move from the bursa into the spermatheca. The sperm often remain viable within the female's spermatheca for her entire life, which may be as long as 3–4 years for social insects such as the honey bee queen. The sperm are released from the spermatheca only when eggs pass down the oviduct, so fertilization may occur just before the eggs are laid. A female insect typically has only one spermatheca, but some coleopterans have two, and there are three spermathecae in some dipterans. These multiple storage organs may allow the female to segregate and manipulate the sperm. The spermathecae vary considerably in their overall structure from insect to insect, but as outgrowths of the median oviduct, they are all ectodermal in origin and produce a cuticular lining. They generally arise from the median oviduct near or on the genital chamber. The spermathecal sac (or receptaculum seminis) is connected to the genital chamber by a secretory duct (or ductus seminalis) through which the sperm are discharged.

Publisher Summary This chapter describes the oviposition behavior. Oviposition behavior comprises one of the final steps in insect reproduction. It involves the deposition of the mature egg outside the body of the female and includes a series of behavioral and physiological events that begin with the movement of the egg through the oviduct and end with the placement of the egg on a substrate that will support the development of the larva. Specialized behaviors and structures on the female allow her to place the eggs within a protected environment during oviposition. In most species, ovulation and oviposition may be part of a continuum, but in others, they may be separate events in which the eggs are retained for a variable period between ovulation and oviposition. Some species of ovoviviparous cockroaches retain within a brood sac eggs that have been ovulated, until the female finds a suitable place to lay them. The nymphs hatch inside the female. The viviparous tsetse that retain and nourish their larvae after they have hatched from the egg, larviposit late instar larvae that pupate after they burrow into the soil.

Publisher Summary This chapter discusses the reproductive system in male insects. The basic component of the male reproductive system is the testis, suspended in the body cavity by tracheae and fat body. The more primitive apterygote insects have a single testis, and in some lepidopterans the two maturing testes are secondarily fused into one structure during the later stages of larval development, although the ducts leading from them remain separate. However, most male insects have a pair of testes, inside of which are sperm tubes or follicles that are enclosed within a membranous sac. The follicles connect to the vas deferens by the vas efferens, and the two tubular vasa deferentia connect to a median ejaculatory duct that leads to the outside. A portion of the vas deferens is often enlarged to form the seminal vesicle, which stores the sperm before they are discharged into the female. The seminal vesicles have glandular linings that secrete nutrients for nourishment and maintenance of the sperm. The ejaculatory duct is derived from an invagination of the epidermal cells during development and, as a result, has a cuticular lining. The terminal portion of the ejaculatory duct may be sclerotized to form the intromittent organ, the aedeagus. The remainder of the male reproductive system is derived from embryonic mesoderm, except for the germ cells, or spermatogonia, which descend from the primordial pole cells very early during embryogenesis.

Publisher Summary This chapter focuses on the hormonal control in reproduction process of male insects. Egg production is cyclical and the experimental manipulation of hormones at key developmental periods has a direct effect on the number of offspring that females produce. This has not been the case for the males of most insect species in which the production of sperm may begin early during the larval, nymphal, and pupal stages and often continues throughout adult life. The development of the testes during metamorphosis and their synthesis of proteins respond to the insect hormone 20-hydroxyecdysone (20HE). In the reproductively mature male, apical stem cells at the anterior end of the testes are surrounded by cyst progenitor cells and specialized non-dividing somatic cells, known the hub, that together form the stem cell niche. Signaling from the niche regulates the renewal, maintenance, and survival of the stem cells. The stem cells divide mitotically to produce a gonialblast that becomes surrounded by a follicular cyst produced by the cyst progenitor cells. The development and maturation of spermatozoa occurs within these cysts. Given the vastly different hormonal conditions that exist during the immature and adult periods, unifying scheme for the control of male spermatogenesis has not been forthcoming. There are only a few examples of hormones controlling the reproductive processes of male insects.

The effect of mating on egg development in starved Aedes aegypti mosquitoes was examined. Mated females that were maintained on water for 4 days before ingesting blood were more likely to develop eggs than were unmated females. The implantation of a single male accessory gland from either A. aegypti or A. albopictus males increased the proportion of unmated females that developed eggs, but glands from A. bahamensis or A. taeniorhynchus did not. Male accessory gland homogenates from A. aegypti were also active when injected at concentrations of 0.02 gland equivalents or greater, but heating them destroyed their activity. When a small blood meal was supplemented with saline to increase abdominal distention, only mated females responded to the distention with increased egg development. Juvenile hormone topically applied to sugar-deprived, blood-fed unmated females also enhanced egg development. If males were starved for 3 days before they mated, they were less likely to stimulate oogenesis in sugar-deprived blood-fed females than if the males were maintained on sucrose before mating.

Publisher Summary This chapter describes the physiology of the respiratory systems in insects. Although there are many striking similarities between the physiological systems of vertebrates and those of insects, one conspicuous difference is in the way that oxygen is brought to the cells. It was Malpighi in 1669 who first identified the series of branching tubes that bring oxygen directly to tissues, a departure from the system in vertebrates where air is regularly and often visibly drawn into the body to oxygenate body fluids that are then circulated to all cells. The hemolymph of the insect circulatory system is responsible for bathing all cells and allowing them to exchange nutrients and metabolites, but the transport of oxygen to the cells occurs through a tracheal system where external surfaces are invaginated into the body cavity to provide an oxygen pipeline from the outside. Although the tracheal system provides an enormous surface area that is permeable to both water and oxygen, water loss by this route is minimal because the system is only open to the outside at the small area that the spiracles present to the environment. Cutaneous respiration occurs in some apterygotes, aquatic insects, and endoparasites. Rather than the continuous ventilation expected by diffusion, a periodic cycle of gas exchange occurs in many insects when they are resting, known as discontinuous gas exchange cycles. Aquatic insects evolved from terrestrial ancestors, and various adaptations have been necessary for them to return to the water, such as cutaneous respiration, plastron respiration, spiracular gills, development of tracheal gills, and evolution of hydrofuge surfaces in insects that retained terrestrial tracheal system.