The second stage of the flea life cycle begins when larvae hatch from eggs. Larvae make up approximately 35% of the total flea population in an infestation. Larval cat fleas are rarely seen by pet owners because they live deep within carpet fibers. In a home environment, the larvae will fully mature and begin pupating in 7 to 11 days. Cat flea larvae do not bite hosts, but instead scavenge for fecal blood and eggs in their environment.
Img 1: Flea larva hatching from its egg, marking the beginning of the second life cycle stage.
animated gif from: Cat Flea Biology
Cat flea larvae are approximately two millimeters long when they first hatch from eggs. Full-grown larvae measure four to five millimeters in length. For comparison, a piece of medium grain rice is five millimeters in length.
Cat flea larvae measure 2 to 5 mm in length. The larval body is a white color. After consuming fecal blood, their guts will turn a red color and can be seen through their semi-transparent body.
Newly-hatched larvae are a translucent, white to off-white color. However, once a larva begins feeding on blood feces, its gut will turn a dark red color. The red digestive track becomes clearly visible through its semi-transparent body. This gives the whole larva a darker, brownish color. Prior to pupating, third instar larvae will shed their red gut contents, and will appear a solid white color again.
Img 2: A cat flea larva under a microscope with visible bristles, head, mandibles, anal struts, and red blood in its digestive track.
photo by: Kalumet
Vid 1: Video of a cat flea larva crawling around a dog’s bed. See the pointing finger for a size comparison.
The larval body is slender and appears like a worm or a maggot. The body is comprised of 13 segments. Three are thoracal (breast) segments and ten are abdominal (belly) segments. Thin, hair-like bristles sparsely cover each bodily segment as well as the head. Two caudal hooks, called anal struts, are located on the end of the posterior segment. Flea larvae do not have legs or other appendages (apod).
Img 3: Illustration of a newly-hatched cat flea larva with 13 body segments, white color, bristles, head, and exaggerated anal struts.
image by: FleaScience
The larval head is distinct from the rest of the body. It sclerotised (hardened) and appears a darker yellow-brown color. An egg burster spine located on the head helps the larvae escape from their eggs (see Img 1). This egg breaker is lost during the first molt. The mouth-parts of flea larvae consist of mandibles and mandibular teeth which are well-adapted for biting and chewing. The larvae do not possess eyes.
Locomotion is facilitated when a larva contracts and relaxes its skin tube muscle on a dry surface. Using its labrum (mouth) and nearby bristles, the larva attaches itself to the surface. Once braced, the larva brings its back-end forward to fasten its anal struts to the surface. The mouth then relaxes its hold and the larva darts forward in a rapid wriggling motion. This method of movement allows the larvae to move quite rapidly.
Img 4: A 3rd instar cat flea larva moving using its labrum (mouth).
animated gif from: Cat Flea Biology
Although mobile, flea larvae will rarely travel far from the point of eclosion. In carpets, 78% of cat flea larvae have been observed to stay within 15 cm from where they hatched. At a maximum, a larva was seen to migrate 46 cm. In a similar study, less than 15% of larvae moved more than 20 cm before they pupated.
Flea larvae will rarely travel further than 15 centimeters from where they hatch.
Larval movement is strongly influenced by the presence of food. When flea larvae hatch in areas without food, they will travel to fulfill their necessary dietary requirements. One study found that 50% of larvae will travel more than 90 cm to find a source of nutrition.
Cat flea larvae will orient themselves in response to certain stimuli. They are negatively phototactic and actively move away from sources of light. Even when confined to sand, the larvae will burrow an average of 2.36 mm to avoid light. The larvae are also positively geotactic. They move towards the ground in a response to gravity. Flea larvae are positively hygrotactic. This means they are attracted to areas of high humidity. Lastly, flea larvae are thigmotactic. They can recognize tactile stimuli and will react to mechanical contact. Larval movement has been shown to be affected by pedestrian and pet traffic.
Cat flea larvae develop through three distinct stages. The stage duration becomes progressively longer as the larvae become progressively older. During these stages, the larvae are sub-categorized into first (L1), second (L2) and third instars (L3). For clarification, the stages are referred to as stages, and the physical forms of the larvae are called instars.
Img 5: Illustration of a cat flea larva as it develops through 3 instars, progressively growing larger and obtaining a red gut once feeding begins.
image by: FleaScience
Flea larvae molt twice, once between each of their three stages (L1 → L2 and L2 → L3). The larvae must molt in order to continue developing. This is because their external cuticle locks them into a fixed size and prevents growth. The periodic ecdysis (moulting or shedding) of the cuticle is the evolutionary solution to this growth problem.
Each larval instar grows progressively larger as a result of molting. However, aside from an increase in size, all three instars are morphologically similar and look alike. There is no simple method to distinguish between instars.
Cat flea larvae will fully develop in seven to eleven days in the average home environment. Modern carpeting in homes creates a favorable environment for larval development. In optimal conditions, cocoon formation begins in as little as four to five days. This rapid development occurs in humid, warm environments that contain an abundance of food. In a poor conditions, it can take larvae 28 to 45 days until they begin pupating. Prolonged development is often the result of low humidity, cool temperatures, oxygen tension, and food shortages.
In the average carpeted home, cat flea larvae will complete their development in 7 to 11 days.
Relative humidity (RH) and ambient temperature are the two most important abiotic factors which regulate larval growth rates. However, overall environmental conditions are somewhat irrelevant because cat flea larvae develop in protected microhabitats. These niches have their own sheltered microclimates where humidity is maintained at a high level and fluctuations in temperature are moderated.
Fig 1 Environment conditions required for flea larvae to survive, and days it takes the larvae to fully develop from egg to cocoon.
A relative humidity between 75% and 92% is optimal for the development of immature fleas. One study found that time to cocoon formation took ten days at 50% RH and only five days at 90% RH. Another study provided similar results. Larvae raised at 50% RH took twelve days to develop, and larvae raised at 65% to 85% RH only took four to five days. One study found a slightly higher optimal relative humidity. A relative humidity between 80% and 90% led to the highest number of larvae surviving. Flea larvae raised in a high relative humidity will also develop into larger adults. Above 75% RH, cat flea larvae are able to actively uptake water from the air.
Fig 2 Days it takes (y-axis) for 50% of flea larvae to form cocoons at different ambient temperatures (x-axis) while relative humidity is held constant at 75%.
The optimal ambient temperature for larval development occurs between 80.6°F (27°C) and 89.6°F (32°C), when relative humidity is kept constant at 75%. Under these conditions, 88% of larvae survive to complete development. Some in as little as five days. When temperature is lowered, time to cocoon formation progressively increases. At 55.4°F (13°C) it takes 28 days for larvae to fully mature (see Fig 2). Another study found similar results, with optimal temperatures occurring between 77°F (25°C) and 86°F (30°C). One study found that 99% percent of larvae will survive at 90°F and 80% RH. Some reaching maturity in as little as four days.
Malnutrition caused by food shortages can result in the larvae taking up to twice as long to develop compared to those which are fully nourished. In addition, malnourished larvae will produce slightly smaller fleas.
Of all the life stages, larvae are the most vulnerable to desiccation (drying out). This makes them highly sensitive to humidity. Relative humidity extremes occur at 50% and 95%. 100% of flea larvae will die when exposed to any RH below 50%, regardless of temperature. A different study concluded that larvae could tolerate no less than 60% RH. However, this low-end extreme is in conflict with an older study which reared larvae at 50%, 55% and 65% RH. Survival was 24%, 77% and 91%, respectively (see Fig 3). At the other extreme, flea larvae are unable survive in saturated air due to the substrate and food growing mold.
Flea larvae are able to survive in a very low relative humidity (12% RH) if they are living on a moist substrate (1% to 20% soil moisture).
Fig 3 Percent of flea larvae that survive (y-axis) at different increments of relative humidity (x-axis) while ambient temperature is held constant at 90°F.
Flea larvae can only tolerate ambient temperatures between 55.4°F (13°C) and 95°F (35°C). However, one study determined that the low-end temperature was a bite higher, occurring at 59°F (15°C). At 50°F (10°C), flea larvae will perish within ten days of hatching from eggs. At 75% RH and 46.4°F (8°C), 65% of larvae will die within ten days, and 100% will die by day 20. At 37.4°F (3°C), 37% of the larvae die on day one, and 100% will die within five days. At the other extreme, temperatures exceeding 95°F (35°C) are fatal to flea larvae. At this high temperature, 30% of the larvae may survive long enough to form cocoons, but they will never emerge to complete their development into adults. Outdoors, there is a complete mortality rate (100%) when larvae are exposed to this kind of heat for more than 40 hours a month.
Unfed larvae will die from starvation within three days if they are unable to locate food.
Flea larvae develop in protected microenvironments which are dark, warm, moist and contain an abundant source of food. Eggs must fall onto substrates with these specific habitat requirements to successfully hatch and continue maturing into adults. Viable zones for the larval development are not widespread around home premises.
Since flea larvae do not move far from where they hatch, where they live is largely determined by where eggs get dispersed. Fleas lay their eggs directly on the host animal, but a majority of fall off within a few hours. Where the eggs drop off depends upon the movement range and grooming habits of the host. A majority of the eggs accumulate around areas frequented by resting pets. One study found that 47% to 50% of flea larvae were located where a cat spent 90% of its time.
Img 6: Flea larvae living within carpet fibers. This is the most common place to find larvae in homes.
animated gif from: Fleas — Michael Dryden, DVM, MS, PhD
In homes, around 83% of fleas will be found developing deep within carpets. Modern carpeting has a deep pile which blocks out desiccating air flow and sunlight, accumulates food, and provides a harborage that protects the larvae from insecticide penetration and mechanical damage (e.g. vacuuming). Flea larvae are also commonly found living in the crevices between hardwood flooring. Other viable development locations include rugs, pet bedding, underneath furniture, folds of a sofa cushion, and dirt-floor basements.
Indoors, flea larvae can be found in carpets, rugs, sofa cushions, hardwood floor cracks, under furniture, and in dirt-floor basements.
Flea larvae are much less likely to suvive outdoors. One study only found fleas in the yards of 2 out of 45 infested residences. Flea larvae will quickly die in open sun-exposed areas because of the low humidity and high heat. Viable larval development zones outdoors must have shade, high humidity, and wind protection. In addition, the substrate also must be relatively dry to support flea larvae. If the soil is too moist (above 20%), then there is a high mortality rate.
Outdoors, flea larvae can be found in thick vegetation, organic debris, flower beds, crawl spaces, dog houses, under porches, and in feral animal nests.
Outdoors, it is most likely to find flea larvae living beneath dense mats of ground cover. These areas must provide over 45% RH, even during the hottest part of the day. The larvae will burrow under organic debris such as grass, soil, branches or leaves. Larval activity occurs in the upper few millimeters of soil. Other locations in yards where flea larvae may be living include doghouses or pet shelters, flower beds, under porches, in crawl spaces, and in feral animal nests.
Cat flea larvae do not have a direct parasitic relationship with hosts. The larvae neither live on hosts, nor bite hosts for blood. The mandibular mouth-parts of flea larvae are well-adapted for biting and chewing solid food sources. This allows the larvae to be free-living scavengers. The larvae will graze for food in their local environment.
At least two nutritional components are needed for proper larval development. Both components are parentally provided by adult fleas in the form of fecal blood and conspecific eggs. Dietary studies have shown that developmental failure occurs when larvae consume only eggs or only fecal blood. However, when the larvae consume both dietary components, almost all the larvae will rapidly develop and emerge as healthy adults.
Cat flea larvae require two nutritional components in their diets: adult fecal blood and conspecific eggs. Consuming only fecal blood or only eggs will lead to developmental failure.
The primary source of nutrition for all larval instars is fecal blood (“flea dirt”) that is naturally excreted from adult fleas. This feces provides the larvae with their minimal nutritional requirements. The larvae will fail to develop into adults without it. Since the larvae require host blood for nutrition and survival, they can be classified as hematophagous obligate parasites, though it isn’t a direct relationship.
Adult fecal blood contains iron-rich hemoglobin, which flea larvae require for normal growth and sclerotisation as adults. The feces also has all of the essential proteins needed for larval development. The feces of adult fleas is composed almost entirely of host blood, with very little digestion taking place. The partially-digested feces contains 83% to 90% of the protein from the original blood, and 44% to 80% of the original ions (potassium and chloride).
Img 7: A cat flea larva feeding on a spherule of dried adult fecal blood (“flea dirt”).
animated gif from: Cat Flea Biology
The large quantities of undigested excreta falling into the environment result from adults consuming much more blood than they can use. On average, adult fleas consumes seven µl of blood per day, and excrete 0.77 milligrams of feces per day. The excreted feces dries and breaks apart into reddish-black pepper-like specks. Adult males and females both produce fecal blood, and the excreta contains the same amount of protein regardless of gender. However, females produce a greater overall volume of feces.
Since larvae rely so heavily on fecal blood, it’s essential for it to fall in the same locations as eggs. And it does. Fecal blood and eggs often drop off hosts at the same time, resulting in similar spatial distributions within host habitats. This leaves a protein-rich food source available to larvae immediately upon hatching.
Fecal blood and eggs residing in the same location is partly due to that fact that females require a blood meal before every egg deposition. However, to a larger extent, it is believed to be an elegant adaptation of flea saliva. After a feeding, flea saliva causes skin irritation at the bite location. Cats and dogs then scratch themselves and dislodge both eggs and “flea dirt”. Eggs and fecal blood are most commonly found together in areas where pets rest and groom themselves. The eggs are smooth and readily fall off hosts at any time. However, the fecal blood is irregular-shaped and usually only gets dislodged from a pet’s haircoat during grooming.
When adult fleas feed, their saliva causes an allergen response at the bite location. The itchy sensation results in dogs and cats scratching themselves and dislodging both eggs and fecal blood at the same time. When larvae hatch, they immediately have their primary food source available.
A diet of fecal blood alone is not sufficient for larval development. One study showed that 100% of larvae will fail to develop into adults when fed cat blood alone. Another study corroborated this conclusion, showing a high mortality rate for larvae which solely consumed blood. Older studies, which are now refuted, concluded that fecal blood alone provided enough nutrition for larval development. However, it was later discovered that larvae will feed on fertile and non-viable conspecific eggs. It is theorized that the larvae in these older studies, without the researchers knowing, supplemented their diets by consuming egg chorions.
Flea larvae have been observed to have a preference for remaining in locations which contain both feces and eggs. One feeding trial showed that a single flea larva will consume 7 non-viable eggs. Another study found that each larva would consume up to 21 dead eggs. It has been theorized that newly-hatched larvae may prefer the soft eggs to hardened blood because they are easier to consume. In addition, it is possible that the eggs have a higher nutritional value.
Along with eggs, larvae may cannibalistically feed on younger or injured larvae. They have also been observed to consume the shed larval casings from previous molts. It is believed that third instar flea larvae may feed on naked pupae (no cocoon) as well.
Flea larvae will sometimes cannibalistically consume younger or injured larvae, naked pupae, and shed larval casings.
In the past, it was believed that cat flea larvae were detritivores and would feed on proteinaceous organic debris found within their local environment. Some examples of this detritus includes decaying vegetable matter, feathers and cat feces. This theory has been refuted. These potential food sources are of no nutritional value to flea larvae at all. In feeding trials, larvae did not consume any of this organic matter. While host-associated materials (e.g. hair, dust, dead skin, fungi and microorganisms, dandruff, and food particles) are likely to be found in larval environments and possibly consumed, the benefit it has to larvae is minimal.
Cat flea larvae do not feed on organic debris in the environment. Though they were once thought to.
In laboratory experiments, flea larvae are sometimes fed blood combined with brewer’s yeast or Purina dog chow. A large percentage of larvae will complete development with these artificial supplements.