The biological characteristics and life cycle of fleas

Ⅰ. Understanding Fleas

Fleas are a group of around 2,500 species and subspecies, distributed across 220 genera, but only a relatively small number of them are important pests for humans. Approximately 94% of species parasitize mammals, while the rest infest birds. Fleas are found globally, although many species are distributed in more restricted areas.

Adult fleas are typically oval-shaped and relatively small in size. Their bodies are flattened from the sides, and their color ranges from light brown to dark brown. They have no wings but possess three pairs of strong legs, with the hind pair especially adapted for jumping. Most of the body and legs are covered with bristles and tiny spines.

The head of a flea is roughly triangular in shape and features a pair of prominent eyes (some species are eyeless), as well as a pair of short, three-segmented, or somewhat rod-shaped antennae, located in the depressions behind the eyes. The mouthparts point downward. In some species, the lower edge of the head is lined with large, developed, tooth-like spines, known as "cheek brushes" or "cheek hooks."

The flea's thorax is divided into three distinct segments: the prothorax, mesothorax, and metathorax. The posterior edge of the prothorax (the back portion of the prothorax) may have a row of tooth-like spines, forming a "prothoracic brush" or "prothoracic hooks." Some genera lack these thoracic brushes and cheek brushes, and are called "brushless fleas." In other species, the flea may have both types of brushes, or only one of them. It is never the case that a flea will lack the prothoracic brush while having the cheek brush. The flea's abdomen contains a segment called the "mesopleuron," located above the middle pair of legs.

Ⅱ. Digestive System of Adult Fleas

To understand the role of fleas in transmitting diseases like the plague, it's essential to know their digestive system and how they feed. During feeding, fleas inject saliva containing anticoagulants into the host's bloodstream. The blood is drawn through the mouth and esophagus into the expanded foregut, which contains numerous (250–450) backward-pointing spines. It was once believed that these spines prevented the blood from flowing back into the esophagus. However, recent experiments suggest that the backflow of blood has little to no impact on disease transmission.

Finally, the blood enters the relatively large stomach (midgut), where digestion occurs. The hindgut is connected to a small, enlarged rectum, which contains rectal glands that absorb water, allowing the feces to be expelled in a dry form.

Ⅲ. Life Cycle

Both male and female fleas feed on blood, and thus both play an equal role in disease transmission. Here, we will describe the life cycle of fleas that feed on humans or animals (such as dogs, cats, and rodents).

A pregnant female rodent flea leaves its host and lays eggs in the debris accumulated in the host's habitat (such as rodent burrows). Fleas that feed on humans or domestic pets (such as cat fleas) lay eggs on the host, but because the eggs are not sticky, they quickly fall off, typically landing in places where the host frequents (like on the pet's bedding or resting areas). The eggs are tiny, oval-shaped, white or yellowish in color, and have no distinct patterns on their surface. Adult fleas typically live for 10 days to 6 weeks, but in some cases, they can survive for 6–12 months or even longer. A female flea may lay between 300 and 1,000 eggs during her lifetime, usually in batches of 3 to 25 eggs per day. Eggs usually hatch in 2 to 5 days, although this depends on the flea species, temperature, and humidity.

A legless larva hatches from the egg. It has a small brown head with a pair of very small antennae and is followed by 13 segments that are light brown and relatively uniform. The posterior edge of each segment has a ring of bristles. The final segment ends with a pair of finger-like abdominal protrusions, known as anal papillae. These papillae and the bristles on the segments help differentiate flea larvae from other medically significant insects.

Flea larvae are highly active, avoid light, and often hide in cracks, crevices, garbage piles on floors, nests, or burrows. Occasionally, larvae may also be found on humans wearing dirty clothing or on beds. Flea larvae can feed on any organic matter, but to successfully mature into adult fleas, many species seem to require ingesting partially digested blood from adult fleas (i.e., adult flea feces). The larval stage usually lasts 2 to 3 weeks, but it can extend up to 200 days or more, especially under unfavorable conditions such as food scarcity or low temperatures. Mature larvae are about 4–10 mm long. Unlike adults, larvae will die if the relative humidity is too low or too high.

At the end of the larval stage, the larvae spin a silk cocoon, which is coated with dust and debris, including tiny grains of sand that it collects from the environment (such as the host's home). As a result, the cocoon is often difficult to distinguish from its surroundings. After about 2 to 3 days of being enclosed in the cocoon, the larvae transform into pupae inside. Adults typically emerge from the cocoon 5 to 14 days later, though this period depends on the environmental temperature and external stimuli. Usually, the stimulus for hatching is the vibrations caused by activity of the host in the home, burrow, or nest. However, if the animal habitat or home is unoccupied, adults may remain in the cocoon until the space is reoccupied. For some species, the emission of carbon dioxide by the host or increased seasonal humidity can also trigger pupal hatching.

Without an appropriate host, certain flea species can survive inside the cocoon for 4 to 12 months, sometimes even longer than a year. This explains why people who move into a building that has been vacant for several months may suddenly be confronted with large numbers of newly hatched, extremely bloodthirsty fleas that are seeking their first blood meal.

The life cycle from egg to adult flea emergence can take as little as 2 to 3 weeks in some species under optimal conditions, but for most species, the lifecycle takes much longer and may extend over several months.

Fleas typically hide in the host's fur or feathers, or on people's clothing, and even in bedding. While feeding, fleas excrete feces composed of partially digested blood and excess blood from previous meals. This semi-digested and nearly undigested blood mixture often leaves marks on the clothing and bedding of people heavily infested with fleas.

Some fleas are capable of surviving relatively dry conditions and prolonged periods of hunger. For instance, in the absence of an appropriate host, fleas can survive for up to six months or more. However, cat fleas and dog fleas typically die within 10 days of leaving their host. When away from the host, fleas tend to jump in search of a new one. Some fleas can jump up to a vertical height of 20 cm. This impressive jumping ability is due to an elastic protein called resilin, which is highly flexible and can be compressed. The rapid expansion of this compressed state generates the necessary force for the jump.

In conclusion, fleas are annoying but difficult-to-eradicate parasites. Understanding their characteristics and life cycle can help in effectively controlling and preventing infestations.