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Part of the tracheal system in the abdomen of a cat flea, x600.
The Life Cycle of the Flea.
Fleas are described as wingless parasitic insects, strongly laterally flattened, with penetrating mouth parts adapted to sucking blood from a mammal or bird host. They spend only part of their life cycle on the host, the larval and pupal stages taking place in the host's immediate surroundings. The adult stage lives mostly on the host animal, feeding and mating.
The cat flea is the most common cause of flea infestation of human houses in temperate climates. Newly laid eggs fall out of the cat's fur into its surroundings -- carpet, bedding and other resting places. Dried flea excrement and cat skin flakes also accumulate in these places, providing food for the flea larvae when they hatch.
The larvae mature over a period of seven or eight days, then form tiny silken cocoons, camouflaged with debris. The pupae within normally develop into adults in a week or so, but the young fleas can lie dormant for long periods, emerging as much as months later when vibrations in their vicinity signal the arrival of a potential new host. They then quickly emerge and leap towards the cause of the disturbance.
If the cause of disturbance happens to be a human, visiting a house previously the home of a cat or cats after a period of being unoccuppied, the result is often instant massive flea infestation of the visitor.
Here is an
early illustration of the stages in the flea's life cycle.
The Larval Stage.
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This is what you see when you shake out the cat's bedding over a sheet of newspaper, collect the shaken-out particles, and place a little on a microscope slide -- numerous larvae, unhatched eggs, egg shells, dried flea faeces, cat hairs and skin flakes.
Flea faeces consists largely of undigested blood. When the adult flea gorges itself, a droplet of blood forms at the tip of its abdomen. This dries in the cat's fur, eventually falling into its bedding. It forms the principal food for these recently hatched larvae, whose digestive systems are coloured bright red with their first meals.
Rheinberg: x50.
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A single cat flea larva. It has clearly begun feeding even before it has completely shed its egg shell, which still grips its rear section. The red object below is dried flea excrement, consisting largely of undigested cat's blood. To the lower right, partly out of the picture, is a flake of cat's skin, which also provides nourishment for the growing larvae.
An unhatched egg (middle right) is oval and completely smooth, allowing it to easily fall out of the cat's fur into the bedding, where hatching takes place.
Rheinberg: x100.
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The Adult Cat Flea.
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An adult female cat flea. The smooth shiny exterior, flattened from side to side, with numerous backward-pointing spines and strong legs adapt the flea well to burrowing forward in the cat's fur, making it difficult to sieze or remove.
The legs are powerfully muscled to enable the enormous leaps for which fleas are justly famous. Relative to their body length, fleas are the best jumpers on the planet. They tumble over and over in their trajectory, and the outwardly turned claws on each leg maximize their chances of lodging in the fur of another host.
Rheinberg: x60.
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Head of a female cat flea. Not all fleas have eyes, and those which do have only simple eyes (ocelli) and never compound eyes. The cat flea possesses ocelli, not clearly visible in this picture. The genal
 comb of dark, flattened bristles surrounding the mouthparts is not found on all fleas, and is useful in identification.
A cluster of spines associated with the first joint of the foreleg can be seen directly below the eye, at the centre of the picture.
The mouthparts are partly visible below the genal comb to the left.
Rheinberg, x150.
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This picture shows the genal comb, the mouth parts and part of the first leg segment of a front leg.
The flea's mouth is a complex structure, and the following text won't make much sense unless you refer to
this diagram, which shows how the parts fit together.
None of the mouthparts are themselves tubular. The ducts which allow blood to be drawn into the flea's digestive system and saliva to enter the puncture wound are only created when all of the parts are slotted closely together to form them.
In the picture, the uppermost part, seen separate from the others, is the epipharynx, which is almost tubular, and is the innermost element during the feeding process. The maxillary palps are the segmented structures lying almost vertically at the centre of the frame. The small spines at the end of the remaining bundle mark the ends of the labial palps.
The fine sharp teeth of the laciniae can also be seen. These are the mouth parts which do the actual piercing.
Darkfield: x150.
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The abdomen of a female cat flea in the genital region.
Rheinberg: x200.
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Another picture of the abdomen in the genital region.
An array of strong bristles is located on either side of the orifice through which the eggs deposited.
Rheinberg: x200.
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The first two segments (coxa and trochanter) of one of the flea's powerful rear legs. These segments are much more developed in jumping insects, and particularly so in the flea. The picture shows an air tube (trachea) passing through the hollow leg joint, and branching into finer tubes (tracheoles) which deliver oxygen to the leg muscle.
Darkfield: x150.
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Another specimen showing the trochanter of the rear jumping leg. This picture was taken some hours after the flea was killed in alcohol solution, and the finer tubes of the air system (tracheoles) have filled with fluid from the flea's tissues, causing them to become almost invisible.
The muscles, which are attatched to the inside of the chitinous exoskeleton, are attatched at their other end to the tendon (above the trachea) which runs lengthwise from one leg joint to the other at the opposite end of the segment.
Darkfield: x200.
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A picture in the region of the thorax and upper leg segments of the cat flea, showing the second thoracic spiracle. This is one of four(?) spiracles in the thorax, which allow the intake of air to the organs of the thorax, and the exhaust of respiration products.
Darkfield: x200.
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The transparency of the flea's abdominal cuticle, its immersion in aqueous alcohol solution, and the use of intense darkfield illumination allows the imaging of the main conduits of the tracheal system lying beneath the abdominal surface.
The openings of three of the abdominal spiracles can be seen at the top of the picture between the rows of external bristles. Their connection to the larger trachea of the respiratory system which run the length of the abdomen is clearly seen due to the refractile nature of the air-filled tubes.
The red colour of the background is due to the gut, filled with blood from the flea's last meal.
Darkfield: x300. (76KB).
Here is a diagram showing the structure of a tracheal tube.
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A flea's knee. The thickening of the chitinous exoskeleton in the region of the joint can be clearly seen, as can the tracheole passing through the hole in the joint.
Darkfield: x400.
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One of the tarsal segments of the flea's leg. The tracheole is expanded into a thin-walled bladder or air-sac which exerts a pumping action when force is exerted upon it by the tensioning of the tendon and the action of the muscle.
The way in which the bristles are socketed into the cuticle can also be seen.
Darkfield: x400.
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Another leg segment containing an air sac.
Darkfield: x400.
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The tarsal section of the flea's leg.
The foot terminates in two strong claws which enable the insect to grip the hairs of the cat, and using its powerful legs, equipped with numerous stiff bristles, to force its way deep into the animal's fur.
Rheinberg: x200.
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Photographic Methods.
All of the micrographs of the adult cat flea are of specimens mounted under a coverglass in a solution of 50% isopropyl alcohol in water. The solution is a preservative, and unlike water, is not prone to the trapping of air bubbles. The chitin of the flea's cuticle, whilst coloured, is quite transparent, and the alcohol solution further enhances this quality. Immersion in a liquid also increases depth of focus.
The pictures were shot in an attempt to show a more realistic view of the flea's internal structure and natural appearance, something entirely missing from specimens prepared for the microscope in the traditional way.
The high power pictures showing details of the tracheal system in thorax and legs were imaged using a Russian x40/0.75NA water immersion achromatic objective fitted with an internal stop reducing its aperture to about 0.55NA.
This increases depth of focus, albeit at the expense of reduced definition, and also increases contrast by reducing flare caused by objects lying outside the plane of focus.
The objective has a long working distance, allowing examination of all parts of the flea without the need to compensate for increasing spherical overcorrection with depth, as would be the case with a dry objective.
A Leitz low power reflecting condenser was used to produce the darkfields (black backgrounds), and Rheinberg illumination (blue background) was achieved using substage stops with an Abbe condenser which was water immersed to the slide for the higher power pictures. This reduces flare and increases image brightness.
The exposures were made with electronic flash on Kodachrome 25 transparency film and scanned on a Canon FS4000US scanner.
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