The information on this page is reproduced by permission of May R. Berenbaum, the author of “Ninety-nine Gnats, Nits, and Nibblers”, published in 1989, ISBN 0-252-06027-X by the University of Illinois Press. Permission from the publisher is being sought. For more of her humorous and informative sketches of members of the Class “Insecta” you might like to buy your own copy!
What's Eating You?
Every spring the expression “What’s eating you?” takes on an uncomfortable air of reality. In most parts of the U.S., spring is synonymous with mosquitoes.
Mosquitoes begin life in a surprisingly inoffensive manner. Immature mosquitoes, known as wrigglers, are complete aquatic. Equipped with chewing mouthparts, they spend the better part of their days feeding on bacteria, yeast, protozoans, and other bits of floating debris. After three larval molts, the immatures molt again and enter the pupal stage. Unlike most insect pupae, the comma shaped mosquito pupae are very active and have earned the name “tumblers” for their manner of locomotion. After a few days (the number varies with the species) the pupa gulps air, splits down the middle, and produces an adult mosquito.
Male and female mosquitoes fly to flowers to obtain nectar – so far, no problem. And male and female mosquitoes court and mate in a number of picturesque ways. In some species, males (the ones with bushy antennae) form swarms into which a female must enter. In other species, males orient to certain sound frequencies, corresponding to those made by females. Oftentimes males accumulate in such numbers that they can clog machinery that happens to vibrate at the stimulatory frequency. This is a minor annoyance, granted. But after mating, the female mosquito requires a blood meal to produce eggs, and, again depending on the species, as likely as not, the blood is supplied by a human being. Indeed, the mouthparts, or proboscis, of the female mosquito make up a surprisingly efficient insect imitation of a hypodermic syringe. There are six needlelike stylets; four actually cut the skin, and the remaining two are pressed together to form a groove through which blood of just about any description can be pumped.
If you really want to know what’s eating you, there are some easy ways of distinguishing the major types of mosquitoes, all of which require suppressing the urge to squash the insect in question to pulp on sight. Anopheles mosquitoes, members of which carry malaria throughout the world, generally lay their eggs in freshwater marshes, ponds, and the like. Each egg is equipped with an air sac (or float) to keep it near the surface. Adults generally have a covering of fine hairs on their bodies and have spotted wings. When they rest (taking a break from a heavy schedule of blood letting), they hold their bodies pointed at an angle to the resting surface, butt end up in the air.
Aedes mosquitoes – these charmers can spread yellow fever and dengue, or breakbone, fever – tend to breed in rain pools, flood waters, and salt marshes, though some can breed in trees, pots, and the like. They often lay their eggs on dry soil which floods later; when the waters rise, the eggs hatch. Aedes adults rest with their body parallel to the resting surface, butt end pointing slightly downward. All are equipped with white scales on their bodies, and many have white bands encircling their legs, and proboscis.
Finally, Culex mosquitoes, the ones that carry Dirofilaria immitis (dog heartworm), St. Louis encephalitis, both eastern and western encephalitis, and filariasis, breed in standing water in places like street gutters, polluted ponds, old tires, flower pots, and just about any kind of container that catches water. They’ve even been found breeding in baptismal fonts. Eggs are laid in “rafts” of several hundred eggs stacked in a single layer. The adults rest parallel to the surface, butt end parallel, and generally lack bands on both legs and proboscis. The most common Culex, Culex pipiens, the common house mosquito, actually doesn’t even prefer human blood. Its tastes run more toward songbirds and poultry, but in a pinch it will feed on horses, dogs, cows, and people (not necessarily in that order). In northern climes, the males die at the onset of winter and the females hibernate in tree holes, inside buildings, or in other protected places.
There are other more subtle ways of distinguishing among mosquitoes. The bites of Aedes, for example, tend to hurt more than the bites of Culex (hence the name of the common floodwater mosquito, Aedes vexans, which translates to mean, “disagreeable annoyance”). An alarming closing note – there are over 100 species of mosquitoes in North America alone and over 2,400 worldwide. If you’re thinking of traveling this summer to escape from mosquito torment, you’d better forget it – there are mosquitoes living from the steamy tropical rainforests to the polar ice caps, where they spend the winter in ice. If you want to get away from them all, try heading for another solar system, just to be on the safe side.
If you have an interest in “current” events, then you ought to be familiar with simuliids, or black flies: black flies are in abundance wherever there’s running water with a fast steady current. They’re particularly common in springs in spring. In the water as larvae, or in the air as adults, black flies are hard to mistake for any other insect. The larvae or immature stages pass their time underwater attached to submerged vegetation, trailing roots, and smooth rock surfaces. They attach themselves by means of a circlet of hooks on their tail end. The circlet hooks into a silken pad produced by the salivary glands. The larvae have a similar circle of hooks on their single front leg, which projects out conspicuously underneath their heads. They can locomote in looplike fashion by alternately connecting and unconnecting the two circlets to the substrate. Their most conspicuous feature are the two cephalic fans, or mouth brushes, on either side of the head, which are used to filter flowing water for food particles. Growing black fly larvae aren’t very particular, and, in addition to filtering and swallowing algae, protozoa, and other microscopic life forms, they occasionally intercept and ingest a younger, smaller, less fortunate fellow black fly.
Larvae undergo six to eight molts and pupate. As pupae, they spin a cocoon to undergo metamorphosis. After two or three days of pupation, the adult black fly emerges. Unlike the larvae, adult simuliids are completely terrestrial. The adults are often (but not invariably) black, stout, little flies scarcely more than a tenth of an inch in length. Their overall humpbacked profile with two stiff hornlike antennae conveys the overall image of a miniature buffalo; hence, yet another common name – buffalo gnats.
Males feed innocuously enough on nectar; for females, it’s a different story. Although they are no less voracious than their offspring, they are a little more discriminating in what they eat. Unfortunately for humans who like to go camping, fishing, boating, or otherwise recreating in or near flowing streams, their meal of choice is the blood of warmblooded animals – almost any warmblooded animals. While no species feeds exclusively on humans, many like to vary their diet, and humans can be caught in the crossfire.
Unlike mosquitoes, which have neat syringelike bloodsucking mouthparts designed to pierce capillaries, black flies are “pool feeders” – that is, they lacerate the skin with their sawtoothed mandibles and then suck up blood from the oozing pool that results. The entire process is, as insect blood feeders go, rather slow and inefficient, and black flies can feed at a site for up to fifteen minutes or longer, taking up their own body weight or more in blood. Once at a puncture, they are determined feeders and can be dislodged only with difficulty. After they depart, residual salivary secretions in the wound can cause intense pain, itching, and swelling for some time afterward.
Most female black flies require a blood meal in order to develop and lay eggs, which they do with untempered enthusiasm on rocks, twigs, or vegetation at or just below the water’s surface. Herein lies the problem. When adults emerge each spring, they so in almost plague proportions, to the extent that there are records of domestic animals dying not from loss of blood but of suffocation, with nose, mouth, and breathing passages blocked with inadvertently inhaled black flies. Mercifully, black flies are extremely seasonal. Although they are exceedingly abundant each spring, by midsummer the overwhelming majority of adults are gone, and they cause few problems for the remainder of the summer – which is, of course, peak season for mosquitoes.
Whoever said that good things come in small packages obviously never encountered ceratopogonids. Ceratopogonids – variously known as biting midges, no-see-ums, punkies, gnats, or moose flies – do indeed come in small packages; the smallest ceratopogonid is only about a half a millimeter (about one fifteenth of an inch) in length and can freely pass through screen doors. These very small insects, however, are capable of inflicting very nasty bites and causing pain out of all proportion to their size. The pain, however, as well as the swelling, blisters, and open sores that follow, is just the tip of the iceberg. Their less than desirable habit of feeding on the blood of horses, cattle, sheep, poultry, humans, lizards, and turtles and even the blood in the wing veins of dragonflies is outdone by their habit of transmitting a staggering array of pathogenic nematodes, viruses, and protozoa. Included in this rogue’s gallery are bluetongue of sheep, horse sickness, onchocerciasis, and fistulous withers of horses, Hepatocystis of monkeys, Parahaemopoteus, Leucocytozoon, and Akiba of birds, and even a filarial parasite of frogs.
Until they sink their mandibles into your skin, ceratopogonids are easy to overlook. An adult resembles any of a number of types of little tiny flies, their most distinguishing feature perhaps being a Quasimodolike hump on the thorax that projects over the head. Unlike mosquitoes, their wings are never scaled. Also, in contrast with mosquitoes, whose mouthparts work on the principle of a hypodermic syringe, ceratopogonid mandibles are scissorlike and cut, rather than painlessly pierce, the skin.
Like mosquitoes, however, only the female ceratopogonid sucks blood. She requires a blood meal to produce eggs. Fortunately, the amount of blood taken by even a very large ceratopogonid is small. Culicoides variipennis, for example, can only choke down about .6 mg at a time, only 20 percent of what a self-respecting mosquito would call a meal. Once she has mated and her eggs are fertilized, a female searches out a place to oviposit. Eggs and larvae are extremely prone to desiccation, so immature stages are basically aquatic or semiaquatic, inhabiting a variety of substrates including moist sandy ocean beaches, shores of streams, creeks or rivers, stock ponds, swamps, salt marshes, mud around water tanks, sewage effluent, holes in trees, and even inside pitcher plants. Many of these aquatic larvae – which are slender cylindrical wormlike creatures with a well-developed head and which may or may not have a single proleg (false leg) near the front end – are voracious predators in their own right, consuming aquatic insect eggs, nematodes, and other small life forms.
Culicoides brevitarsus, however, a pest of cattle, stays close to home and breeds in cattle dung, particularly in the wet spongy layers. The eggs of this species are equipped with a layer of hairs that physically trap air and permit respiration in the semiliquid manure. Development in the dung takes anywhere from three to four weeks, and the pupa prepares for adult emergence by anchoring itself into crevices in the dung pat by a pair of long spines at the tip of the abdomen.
An adult male’s first order of business upon emergence is to find a mate, and to do so males of most species join a swarm. Depending on the species, a swarm may form either around a host or form independently of a host. Generally males face upwind and move up and down across any bit of of distinctive ground cover that can serve as a marker. There’s a peculiar sense of urgency associated with male ceratopogonids – a dispersed swarm, for example, re-forms after only ten seconds, and movement in and out of a swarm reminded one entomologist of “rush hour public transit.” This may well be due to the fact that male fertility is a very fleeting thing; potency begins to decline at the ripe old age of 8 hours. Mating of a virile male only minutes old takes less than 8 1/2 minutes but by the time he reaches 24 to 36 hours of age, a male can take over an hour to get the job done. Female ceratopogonids are hardly forgiving in matters of sexual performance. They are known to resist advances by older males by running away, tipping their abdomen, or even kicking violently. In some species, females enter a swarm to capture males and eat them and can occasionally be seen feeding on their own partner while in the very act – a consuming passion indeed.
Despite what you might assume on hearing its name, the best place to find a stable fly is not inside a stable. Unlike its close relative, the house fly, the stable fly almost never lays its eggs on horse manure – or any other kind of manure for that matter. Rather, it lays its eggs on fermenting vegetation such as weeds, lawn cuttings, hay, or seaweeds. Granted, fermenting hay or straw is most likely to be found in or around stables, but stable flies are just as likely to be found breeding on the seaweed-strewn shores of lakes and oceans as they are at Belmont Park or Pimlico. To add to the confusion, the fly you’re most likely to encounter in a stable is actually Musca domestica, which is, of course, the familiar house fly. Moreover, stable flies, or Stomoxys calcitrans, can occasionally be found in houses, where it’s known as the “biting house fly.”
Underlying this etymological confusion is entomological confusion as well. The stable fly is, to the uneducated eye, an exact double of the house fly. Both are greyish flies about one-fourth of an inch long with four dark longitudinal stripes on the thorax. A trained entomologist can recognize a stable fly by its slightly broader abdomen, its habit of holding its wings widely spread at the tips instead of straight back, and its antennae, which are hairy only on the upper side. But even the rank amateur can detect the most important morphological difference between the stable fly and the house fly: while house flies have spongy soft mouthparts which they use to suck up liquified garbage of all descriptions, stable flies have bayonet-like mouthparts which they use to puncture skin and suck blood.
The stable fly isn’t particular as to whom or what it punctures. Stomoxys calcitrans feeds readily on the blood of rats, rabbits, monkeys, cows, horses, goats, sheep, dogs, humans, and even birds and reptiles. Unlike most kinds of biting flies, in which only the females feed on blood, stable flies are affirmative action advocates – both male and female stable flies partake of blood readily.
Female stable flies lay their tiny whitish eggs, curved on one edge and straight on the other, in among spaces in loosely packed vegetation. One female can produce over six hundred in her lifetime. After two to five days, the eggs hatch into maggots, which basically resemble house fly maggots (distinguishing the two involves getting closer than most people care to do). Larval development takes about two to three weeks, after which time the larvae find a dry spot to molt into the brownish puparium. After one to two weeks, metamorphosis is complete, and an adult stable fly, anxious for its first blood meal, can emerge from its puparium in under an hour.
Although stable flies draw blood quickly, using their proboscis like an awl to puncture skin, and can feed to capacity in three to four minutes, they prefer to take a little at a time and change hosts frequently. Stable flies are therefore excellent vectors of disease and can carry the causative agents of trypanosomiasis on their mouthparts. They can cause weight loss and reduction in milk production in cattle not only by blood and tissue loss associated with blood feeding, but also by the constant aggravation and stress they create by persistent attack. So, in a pasture or paddock, stable flies mean unstable cattle or horses nearby.
If you feel ticked off every summer, it’s probably the fault of Dermacentor variabilis. Dermacentor variabilis, known as the American dog tick, is one of the commoner members of the bloodsucking family Ixodidae, otherwise known as the hard ticks. They’re called hard ticks because the members of the family all share a shieldlike plate on their back called a scutum. In Dermacentor variabilis the scutum is white and, for a tick, fairly ornate. If for some reason you’re ever interested in sexing American dog ticks, the male’s scutum reaches almost down to its back end and puts a limit on the amount of expansion it can accomplish while feeding. In females the scutum reaches only a short distance behind the head (or capitulum), leaving them free to swell up to pea-size proportions while feeding.
Probably everyone is acquainted with the feeding habits of ticks – they suck the blood of just about any warm-blooded vertebrate they can sink their hypostome into. Adult Dermacentor variabilis are partial to dogs but won’t pass up a tasty cow, horse, or human, given a chance. The immature stages, on the other hand, are fond of meadow mice and other smaller mammals.
If you’ve ever wondered what happens after a tick feeds, if it’s a female, she generally falls to the ground and in four to ten days lays from four to six thousand eggs. The eggs hatch after about a month, and the six-legged larva, or seed tick, looks for its first meal, often lying in wait on the tips of low-growing vegetation. After it finds a meal it molts into an eight-legged nymph, or yearling tick. The nymph repeats the whole process of finding a host and engorging. It then molts into an adult, finds a host, attaches for a week or two, and mates while feeding (think about that the next time you check yourself for ticks). Dermacentor variabilis is a three-host tick, requiring a different individual host to complete each life stage. The related cattle tick, Boophilus annulatus, in the interests of efficiency is a one-host tick, taking all three meals and completing all three molts on a single host.
Since hosts are hard to come by, hard ticks are accustomed to hard times. Nymphs can live over a year without a meal, and adults can go two years between meals. Lest one feel sorry for a tick having to go hungry, one should keep in mind there’s injury to the insult of tick bites. Dermacentor is an accomplished vector of Rocky Mountain spotted fever, Colorado tick fever, tularemia, bovine anaplasmosis, and possibly Lyme disease, and, even if it isn’t spreading disease organisms, it can cause illnesses such as canine or tick paralysis by injecting salivary toxins.
Although a natural impulse when you find yourself parasitized is to grab the tick and yank it out, it’s really not a great idea. Tick mouthparts are covered with sharp backward-pointing teeth that keep the whole assembly in place until the tick is ready to remove it. Dousing the tick with gasoline or alcohol or holding a smoldering match behind it will disturb it sufficiently to loosen its grasp, whereupon it can be removed with ease and dispatched.** (Don’t, though, douse the tick and ignite it). If you yank without preliminaries, the tick’s head can remain in place while you dispose of the inoffensive body. In place, the head can cause infection or even paralysis. So a tick bite is no cause for panic – stay calm, don’t lose your head, and, by all means, don’t let the tick lose his.
I recently received a concerned email warning that the procedure for removing ticks outlined here is in fact a potentially dangerous one. The following is a safer approach:
A tick should NEVER be dispatched by, “Dousing the tick with gasoline or alcohol or holding a smoldering match behind it will disturb it sufficiently to loosen its grasp, whereupon it can be removed with ease and dispatched.” Doing this can cause the tick to salivate more which could put vectored organisms into the host. The best way to dispatch of a tick is to get a pair of tweezers and grab the tick by the mouth and pull in a slow steady motion. Do not twist the tick or rock it back and forth because it has a greater chance of breaking the head off. A slow steady pull with tweezers should keep the tick head intact and it will not stimulate the tick to produce more saliva. This is very important that you correct at least this mistake in your webpage. Also, you might want to include that if a person thinks that a tick has fed on them, keep it in a plastic bag. This will save a lot of guessing if they are to become sick a few days later by taking that tick to the doctor.
Thanks to Daniel McCoy for this information – May 24, 2008.
Chiggers, also called jiggers, redbugs, and a number of far more unflattering things, are actually arachnids rather than insects. They’re mites in the order Acari. Their closest relatives, who are equally obnoxious, are the ticks. Mites, by and large, are anything but large – they range in size from minute to small, and chiggers are mid-sized as mites go.
Most people acquainted with chiggers have struck up the acquaintance by accident, walking through grass, weeds, or brambles in hot weather. In three to twenty-four hours, ankles, wrists, knees, waists, and any place where clothing contacts the body frequently begin to itch uncontrollably. The unrelieveable itching can persist for a week or longer and is accompanied in time by swelling, redness, pustules, and scabs. This summer delight is a chigger attack. If you aimed a microscope in among the pustules and scabs, you might be able to see a tiny bright red dot about 1/150 of an inch across. That’s a chigger.
Chiggers are the immature stages of several mite species in the family Trombiculidae. Every spring adult mites emerge from their overwintering sites underground and lay eggs. After about a week, the blind baby mite crawls out of the egg and climbs upward, preferably on tall vegetation. Unlike the adult mites, these first stage larvae have only six (as opposed to eight) legs. They suffer from no great reduction in mobility as a consequence; they’re extremely agile and can move very quickly. They’re sensitive to carbon dioxide, because, since all animals breathe and in the process exhale it, carbon dioxide signals the approach of an impending meal. The larvae leap onto any passing vertebrate, and, lest humans feel too persecuted, it should be mentioned that they are equally attracted to chickens, ducks, frogs, toads, turtles, squirrels, cows, horses, rabbits, rodents and a host of other animals. They’re especially partial to snakes, on whom they settle down underneath the overlapping scales (and snakes, it should be said, have no appendages to scratch with either).
Once on their host, chiggers begin to feed. They don’t, as many people believe, actually burrow down underneath the skin. Ewing in 1921 patiently (and courageously) observed twenty-six chiggers on his own skin and noticed that they attach to the skin surface or at the base of a hair. They don’t actually suck blood, either. They simply inject an enzyme-filled fluid that disintegrates skin cells. This soupy mess of cytoplasm and cell bodies is sucked up by the mites. After they’ve eaten their fill, they drop to the ground and burrow into the soil. The swelling and itching are the result of an allergic response on the part of the host to the substances injected by the mites and can begin long after the chiggers have departed.
The immature mites remain in the soil and molt at least one more time before emerging as adults the next spring. Aside from the fact that they produce baby chiggers, the adults are completely harmless to other life forms. They mostly eat decaying wood and insect excrement.
Unfortunately, chiggers are not the only acarine occupants of human skin – the human body is a veritable zoo. Follicle mites, Demodex folliculorum, live in the hair follicles and sebaceous glands around the nose and eyelid. Follicle mites are worldwide in distribution and it’s been estimated that three out of every four people harbor a population on their face. So to three-fourths of you readers out there – my consolations.