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Publisher Summary The metabolic systems of only a small number of insect species have been examined, most often cockroaches, blow flies, fruit flies, or caterpillars. The evidence for the existence of complete metabolic pathways in insects has often been based on the presence of certain key enzymes, reaction end products, or intermediates that may also exist. The processes of food breakdown, utilization, and storage are strikingly similar in insects and vertebrates, as well as in most other living things. However, some distinctive metabolic processes are found only in insects. This chapter describes the alimentary system, digestion, and metabolism of nutrients in insects. The evidence for the existence of complete metabolic pathways in insects has often been based on the presence of certain key enzymes, reaction end products, or intermediates that may also exist in vertebrate systems where the cycles were more completely identified. As in vertebrates, the determination of insect metabolic pathways is complicated by the presence of symbiotic microorganisms that may provide some of the steps missing in the insect, particularly in those systems that require trace components such as vitamins. Some wood-feeding insects depend on their microorganisms to break down the lignin, and many phytophagous species can extend their possible plant hosts by relying on the bacterial detoxification of secondary plant substances. These symbiotic contributions make it difficult to establish whether the metabolic pathways are actually native to the insect system being studied. Many predaceous insects employ extra-oral digestion as a strategy to utilize prey that is too large to be swallowed whole. Furthermore, in anticipation of unfavorable environmental conditions under which an insect population would not survive, many insect species initiate a series of metabolic changes called diapause well before the unfavorable conditions actually occur to survive adverse conditions.

Publisher Summary Communications between two or more whole organisms, both intraspecific and interspecific communications, are absolutely essential for an organism's survival and reproduction. In insects any signal that produces a change in the behavior of another individual will be considered to have been a communication. This chapter reviews some of the ways that insects produce these signals. One way to classify the systems of communication is according to the receptors that are involved in receiving the stimuli. Various chemical, tactile, acoustical, and visual signals have been identified as being the primary means of communication in insects, each with its own advantages and disadvantages. Insects use visual signals primarily to identify food and mates and to orient themselves in the environment. The compound eyes are most important in receiving the visual signals because they have the best resolution of the optical receptors present. Visual signals are often releasers of the highly ritualized and stereotyped insect behavioral sequences. Insects produce sound and vibrational signals through the air, water, or solid substrates, and they receive and interpret these signals by what may be inferred to be hearing. Tactile signals in insects are used for short-range communication, generally for aggressive or sexual encounters. Insects make an extensive use of chemicals, such as hormones or semiochemicals, pheromones, and allelochemicals, which are released into the surrounding medium for communication. They employ chemicals for communicating to find a mate, gather together, provide others of their own species with the location of food, identify nestmates, and defend themselves against predators. Insects live in a complex chemical world, with chemical cues governing much of their behavior and most of their interactions.

Publisher Summary Behavior, like morphological structures, is a phenotype whose development and expression are influenced by both genetic and environmental factors. Behavioral physiology examines how they do it and the physiological reasons for why a particular behavior may be expressed at a particular time. Although usually simple to observe and requiring little in the way of instrumentation to measure, behavior represents the end result of a complex series of nervous impulses and muscular contractions that are initiated in a specific order and that ultimately result in the expression of what we see as a spatial displacement of the insect. In insects, evidence for a genetic basis for behavior is more obvious. One of the best examples of how single genes affect behavior is seen in Drosophila melanogaster, where specific behaviors are programmed by genes that directly regulate the development and function of the central nervous system. The fruitless(fru) gene plays a critical role in establishing the neural circuitry that governs male-specific courtship behaviors in Drosophila. Many insects, particularly social insects, are guided to their nests by physical landmarks, as they are capable of recognizing and remembering environmental patterns. The mechanism of this pattern recognition is most likely retinotopic matching. The nervous system of bees produces, retains, and processes information that is used to identify resources far from the nest. The genes that are associated with particular behaviors may produce their effects by establishing particular neural pathways and also code for the production of hormones that trigger the expression of the fixed action patterns that are already present in the nervous system. Circadian rhythmicity is among the best example of the link between genes and behavior, and insects were among the first animals to have their circadian clocks localized to specific areas of the brain.

Secretions of the male accessory glands of Aedes aegypti (L.) induce gravid virgin females to engage in pre-oviposition behaviour. Normal mating, the implantation of whole glands, or the injection of male accessory gland homogenates, all induced pre-oviposition behaviour in gravid virgin females, but implants of testes did not. Injection of male accessory gland extracts from other mosquito species, including A. albopictus, A. sierrensis, A. taeniorhynchus and Anopheles nr. salbaii into gravid virgin A. aegypti females did not induce pre-oviposition, suggesting that the male components were species-specific. Male accessory gland substances retained their biological activity when stored at 4°C for 24 h or when maintained at −10°C for 2 weeks, but lost their activity after longer periods and were inactivated by heating.

A method is described for using near-infrared reflectance spectroscopy (NIRS) to quantify the amount of blood present in mosquitoes without killing insect. NIRS is a nonconsumptive, instrumental method for fast, accurate, and precise evaluation of chemical composition of material with little or no preparation. The NIRS method accurately predicted bloodmeal size and bloodmeal disappearance in Aedes aegypti (L.) and Ae. sierrensis (Ludlow) mosquitoes.

Both Aedes aegypti and Anopheles gambiae mosquitoes undergo physiological and behavioral changes after the females mate, but unlike Ae. aegypti, the mated state in An. gambiae is not attained through the action of male accessory gland substances. Experiments in which the spermathecae of mated females were manipulated suggest that a spermatheca filled with sperm is responsible for triggering oviposition behavior. An estimated 48% of the previously mated An. gambiae females mated again when the interval between encounters with males was less than 24 h. Unlike male Ae. aegypti that can inseminate up to seven females after their testes are removed, An. gambiae do not store sperm in the vas deferens and without testes would be unable to inseminate any females.

The effect of mating on egg development and the size of the female accessory glands during the gonotrophic cycle of Anopheles gambiae was investigated. Although females that received a measured meal of blood were more likely to produce an egg batch when they were mated, the increased reproduction was not the result of male accessory gland substances. Changes in the size of the female accessory gland were not observed after mating nor at any time during the gonotrophic cycle, but there was a more rapid increase in its size following emergence in mated females.

The process of neurosecretion is an important and widespread method of biological communication among animals. Although insects and vertebrates appear to be very different, neurosecretory mechanisms and the neuropeptides themselves are often the same. The gradual acceptance of neurosecretion as a biological phenomenon, largely as a result of research done with insects, is discussed. Arch. Insect Biochem. Physiol. 53:101–114, 2003. © 2003 Wiley-Liss, Inc.

Females of the afrotropical mosquito species Anopheles gambiae Giles sensu stricto and An. quadriannulatus (Theobald) (Diptera: Culicidae) were studied for the effect of blood meal size and the frequency of blood feeding on reproductive development during the first gonotrophic cycle. To standardize the blood meals, meals were administered by enema in some experiments. The effects of insemination, mosquito size, and metabolic reserves at emergence on egg development were also investigated. Maximum insemination was reached after seven days, varying from 62% in An. quadriannulatus to 95% in An. gambiae and was significantly different (P

Although male accessory gland substances modulate female receptivity in most mosquito genera that have been studied, they are not active in the development of sexual refractoriness in the malaria vectors, Anopheles gambiae or Anopheles albimanus. Neither the implantation of male accessory glands nor the injection of gland homogenates affected the insemination rate of unmated females. Similarly, portions of the male genital tract and spermathecae from mated females were also inactive when introduced into these females. Interruption of nervous transmission from the brain reduced the tendency of females to become inseminated and removal of the terminal abdominal ganglion completely abolished insemination. Oviposition behavior did not occur unless the spermatheca containing sperm was present, suggesting that this may be the mechanism that mediates the switchover to mated behavior.

Autogeny in Aedes albopictus (Skuse) females was increased from 5% in wild-type parental stock to 84% by selection through 20 generations and was decreased to 0% by selection through nine generations. Concomitantly, the mean number of eggs per female that matured autogenously increased from 6.4 to 18.3 in sugar-fed mosquitoes that developed eggs. Although the duration of the larval period of the two strains was not different, autogenous females emerged with greater amounts of metabolizable protein and lipid and total available energy reserves than did anautogenous females, but with lesser amounts of glycogen. Dry weights did not differ significantly. The difference in the energy reserves between strains was slightly more than the total energy invested in 20 autogenous eggs. Autogenous females survived 1.3 d longer than did anautogenous females when starved from emergence.

A semi-isolated cockroach heart preparation was used to rapidly determine the activity of cobra cardiotoxin, monitored as a direct response on heart rate. This preparation produced a dose-response curve in the presence of active cardiotoxin and demonstrated that cardiotoxin retained its biological activity after boiling, although cardiotoxin activity was destroyed by heating in the presence of dithiothreitol. Experiments that cross-linked radiolabeled cardiotoxin to solubilized cockroach heart membranes suggested that cardiotoxin bound specifically to a 59,000 mol. wt membrane protein in this tissue.

The effects of a single sucrose meal or topical application of juvenile hormone III or methoprene on egg development following a 2 μl blood meal were studied in otherwise starved adult Aedes aegypti mosquitoes. Juvenile hormone and sucrose both increased the proportion of mosquitoes that developed eggs if they were administered during a critical period after the blood. Methoprene was effective for a longer period, but eggs that resulted were not viable. At 16 h following the blood meal, both juvenile hormone and methoprene induced significant mortality, but at 24 h, only methoprene had this effect. Additional ingested blood or sucrose could reduce the juvenile hormone-induced mortality. The glycogen content of sugar-fed females after methoprene treatment was significantly reduced, suggesting that treatment with juvenile hormone or its analogue induced vitellogenesis by mobilizing nutritional reserves, or causing mortality when the reserves were depleted. The implications for the involvement of juvenile hormone in the reproduction of mosquitoes following a blood meal is discussed.

Host-seeking behavior in Aedes aegypti is inhibited during egg maturation. The onset of this inhibition is delayed in chronologically old females, and the degree of inhibition is reduced if the mosquitoes have undergone a prior gonotrophic cycle. Results of reciprocal ovary transplants between young and old mosquitoes suggested that the decreased rate of yolk deposition in old females was not solely responsible for these differences in host-seeking behavior. The importance of distinguishing between gonotrophic aging of the ovary and chronological aging of the whole female, and the relationship of these factors to multiple feeding, are discussed.

Pre-oviposition behaviour, the attraction of mosquitoes to oviposition-site stimuli, is induced by a haemolymph-borne substance. A large proportion of gravid mosquitoes was attracted to a solution of methyl propionate in a laboratory olfactometer, but non-gravid females were also attracted if they were first injected with haemolymph from gravid females. Ovariectomized blood-fed mosquitoes failed to respond, but the reimplantation of denervated ovaries or as few as six mature follicles still triggered pre-oviposition after a blood meal.

Mosquitoes maintained on water from emergence were less likely to develop eggs after a blood meal compared to those maintained on sucrose, and those carbohydrate-deprived females that did develop eggs were more likely to approach a host while gravid than those fed sucrose. When small blood meals were ingested, the proportion of carbohydrate-deprived gravid females approaching a host was increased even further. The ingestion of a single meal of sucrose influenced both egg maturation and the behaviour of the gravid mosquitoes, but its effect varied according to when the sugar was ingested. Although the initial step of host-seeking inhibition is ovarian dependent, implantation of ovaries from starved females into sugar-fed ovariectomized hosts demonstrated that this step is not affected by starvation.

Female Aedes aegypti mosquitoes were given 1 μl blood enemas, and their subsequent host-seeking behaviour determined in an olfactometer. Those females failing to develop eggs consistently responded to a host stimulus whenever tested, but inseminated mosquitoes developing eggs were inhibited from host-seeking during the period of egg development. Gravid uninseminated mosquitoes were also inhibited, but not to the same degree as inseminated mosquitoes. Experiments involving surgical manipulations and haemolymph transfusions indicate that a haemolymph-borne substance, present during egg development, inhibits the response toward a host.

The inhibition of host-seeking behaviour that accompanies vitellogenesis in the mosquito, Aedes aegypti , was examined by the removal and implantation of ovaries. Mosquitoes ovariectomized before a blood meal and between 1 and 6 hr after a blood meal responded to a host at 48 hr after a blood meal. However, when ovariectomy was delayed until 10 hr after the meal or later, most mosquitoes did not respond to the host. When a partial ovary was present for only the first 12 hr after a meal, there was no host-seeking inhibition at 48 hr, and only 58% of females with one complete ovary present during this time interval responded. Howver, these same amounts of ovarian tissue inhibited host-seeking when they remained for 48 hr after a meal. Vitellogenic ovaries from donors blood-fed 8–24 hr before, implanted into sugar-fed recipients, did not affect the host-seeking behaviour of these recipients. Ovaries removed and reimplanted before the blood meal inhibited host-seeking at 72 hr after the blood meal only in the absence of oviposition from intact ovaries. It is concluded that 2 humoral factors are involved in the promotion of host-seeking inhibition: the first factor is produced by the ovaries, and after reaching a critical threshold in the haemolymph, stimulates the release of a second factor that acts directly to inhibit mosquito behaviour. An ovary which retains 2 or fewer eggs after oviposition terminates the inhibition via nervous pathways. The role of 20-hydroxyecdysone in the behavioural inhibition is discussed.

Abdominal distention accelerates the release of a factor from the head of blood-fed Aedes aegypti mosquitoes. The critical period during which the head is required for oogenesis following blood ingestion is approx 6 h with a 5 μl meal, but small blood meals of 1 μl require the head to be present for significantly longer. Increasing the abdominal distention by supplementing the 1 μl meal with saline results in a critical period similar to that with 5 μl of blood. The information from the distended abdomen appears to travel via the ventral nerve cord. Transection of the ventral nerve cord prevents oogenesis from occurring after small blood meals, but not with larger blood volumes. Topical application of 100 pg of juvenile hormone III can substitute for the distention message.

Following a blood meal that initiates oocyte development, the host-seeking behaviour of Aedes aegypti mosquitoes is inhibited by a haemolymph-borne factor that is released in response to a humoral signal from a vitellogenic ovary. This inhibition is accompanied by a decrease in the sensitivity of the peripheral lactic acid receptors. Implantation of corpora allata, medial neurosecretory cells, or terminal abdominal ganglia from blood-fed donors could not induce the inhibition in sugar-fed recipients. However, fat body transplanted from blood-fed into sugar-fed females suppressed host-seeking behaviour as well as the sensitivity of lactic acid receptors, suggesting that the source of the behavioural inhibitor is the fat body. Resting-stage ovaries from other mosquito species inhibited host-seeking after the A. aegypti host was fed on blood only if the fat body was activated by the donor ovary.

SUMMARYThe faecal survival ofSalmonella typhimuriumorally administered to restrained American cockroaches maintained on an antibiotic diet was investigated. Significant reductions in median numbers of total aerobic faecal microorganisms were noted in insects fed antibiotics daily, and whenSalmonellawas introduced to this modified micro-environment, it persisted for 44 days in all specimens. Multiplication of the pathogen was also observed in these insects, with numbers often exceeding 103times that of the initial input. This differs significantly from our previous results showing that the pathogen is unable to multiply in conventional cockroaches. Attempts to restore the normal flora by feeding a faecal suspension from untreated cockroaches resulted in a decrease in numbers ofSalmonellaexcreted, but did not result in their elimination. Carcasses of infected cockroaches retained viableSalmonellafor at least 60 dayspost mortem, or 104 days after the infective meal.

The blood meal sizes of field and laboratory mosquito populations attracted to stable traps baited with restrained or unrestrained rabbits were measured. Mosquitoes feeding on the unrestrained host were influenced by its defensive behavior; feeding success was significantly reduced compared to that of females feeding on a restrained host. The amount of blood ingested from the defensive hosts was also significantly reduced, except for Aedes sollicitans which fed fully even when the host was active. The implications of the data for multiple feeding behavior are discussed.

When American cockroaches were fed Salmonella enteritidis serotype Typhimurium, their hemoceles were found to be infected with the bacteria within the 1st hour. To determine the importance of this mechanism of infection in the establishment of a carrier state, the course of Salmonella persistence in the hemolymph was examined. Approximately 80 bacteria were found in the hemocele after 12 hr. The infection persisted for 5 days and was gradually climinated by the insects' cellular defenses. Evidence for cellular elimination is presented in experiments in which approximately 102 salmonellae were injected into the hemocele. The bacteria persisted for 3 days in males and 6 days in females at 22°C; these sexual differences are correlated with a higher total hemocyte count in males. Latex beads with the same inoculum of Salmonella prolonged persistence to 8 days in males. At 27°C, persistence was decreased to 4 days in females. Additional evidence that phagocytes are mainly responsible for clearing the hemolymph is the large number of rods present in the phagocytes of cockroaches injected with salmonellae and their absence in controls.