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Rhantus suturalis larva, head 05-08-2007
enlargement Rhantus suturalis, larva
THE LARVA OF THIS DIVING BEETLE HAS THE TYPICAL FEATURES OF A PREDACEOUS WATERBEETLE LARVAE: the segmented body, the flat head, the crescent shaped jaws, and, last but not least, the enormous gluttony.
rhantus suturalis, young larva 05-08-2007
enlargement Rhantus suturalis
young larva
Each midge larva entering the range of the jaws is grabbed and devoured. According to some investigations up till some 30 a day! They must eat so much in order to be full grown in a short period: only three weeks. In their blind feeding frenzy they even assault family members that don't respond aggressive enough. When one is too obtrusive to another they act like wild cats: after a flashing move they are opposing each other with threatening, wide opened jaws. It's only with tightly filled stomaches and enough living space that cannibalism is not practised. Than they may be hanging out leisurely next to each other in the water plants. Unknowingly they may help each other: practical experiments proved that larvae in groups catch more prey than lonely specimen.
The larva on these pictures are the offspring of the beetle on the previous page. They did grow fast: begin august they hatched, and already on august 24 they were out of the water! Such speed is necessary, because in summertime the pools in which they live often may dry out quick. And so, both gnat larva and beetle larva have to race against the calendar to catch their flight in time.
The legs of the Rhantus larva are partially fringed with hairs. They allow the larva to swim rather well, though not as fast as the beetle. And the larva has a habit of moving over surfaces and waterplants, on which the legs then more or less are scratching.

Rhantus suturalis, young larva gets fresh air 05-08-2007.
enlargement Rhantus larva respirating
Rhantus suturalis, siphon of larva 05-08-2007.
enlargement Detail snorkel and cerci
rhantus suturalis, older larva pending on watersurface 13-08-2007
enlargement detail tail
cerci of Rhantus larva
The larva are able to float in the water while suspended at the tip of their abdomen to the water surface to ventilate their air tubes, as the picture on the right is showing. But mostly they do that while resting on waterplants or on the bottom. To take in air, the larva push the end of their body against the water surface. The two "main airducts" (trachaea) end at the tip of the abdomen in two openings (stigmata) that are brought in contact with the air above the water. In this way the tip acts as a kind of snorkel. Many water beetle larva have two appendages(cerci) at the tail tip. Typically the Rhantus genus has cerci covered with short and/or long "hairs"(setae) over the full length. The tail appendages stabilise and secure the position of the snorkel. The water repellent/adhesive properties of the formerly mentioned body parts build a complex system of physics, all to make effective use of the unvisible watersurface film. Of course the larva has no air supply under wingcases like the beetle has, but nothing more than the amount of air in it's trachaea. Probably it is able to breathe trough the skin, but fact is the younger larva emerge often to the surface to remain there aspirating. The older larva can remain under water surprisingly long, maybe it's thin white belly skin is more penetrable for the oxygen in the water.

Rhantus suturalis, larva with prey 13-08-2007
Rhantus larva with prey
Rhantus suturalis, larva 2 weeks old 13-08-2007
enlargement eating midge larva
Like raging tigers among sheep the voracious larva of Rhantus suturalis are murdering the defenceless little gnat larva, that only can escape by a quick movement. Most of the time the stealth hunting beetle larva is quicker... The victims are pierced and pinned with the merciless, pincerlike jaws. Just like those of the larva of the Great diving beetle larva these jaws have closed grooves, forming small tubes. Through these tubes a poisonous, protein dissolving enzyme is injected in the prey, after which the nutrients thus formed are sucked in. The process of injecting and sucking in is continuously repeated and can be seen through the transparent skin of the larva: a brown fluid is streaming through a central duct, that branches in the head into the jaws, and back again. In that way the first digestion is taking place outside the body of the beetle larva. As a rule the meal is spend with the tip of the abdomen at the water surface. When the mosquito larva is mostly consumed, it seems as though the larva gobbles up some of the food mush from the prey. (The larva of the Great diving beetle is not able to do so because it has a locked mouth slit). At last the empty carcass of the gnat larva is left loose and from the same moment the larva is looking greedy for the next prey.

Rhantus suturalis, larva 2 weeks old 13-08-2007
Rhantus suturalis, larva a few days old 05-08-2007
enlargement
Midge larva 13-08-2007
Rhantus suturalis, larva 2 weeks old 13-08-2007
enlargement 2 weeks old
Thanks to their protein diet the larva grow very fast. They moult three times. On the left, all at the same scale: a two weeks old larva beside one which has hatched a couple of days before; on the bottom picture a midge larva. The young beetle larva is not much bigger than the midge larva, so if it hasn't grown large enough in short time, it will have any chance of becoming a prey itself, as you can see on a photo on the web link below. At right: two pictures of the same older larva. The fat, white little bellows of full grown larvae are quite conspicuous when they turn over on their backs when swimming. After three weeks they are 15 millimeter long and full grown, they then stay longer under water and are less active moving.

Rhantus suturalis larva 13-08-2007

Rhantus suturalis, larva about to pupate 13-08-2007
Rhantus larva in pupation hole
At last the larva climbs on the border and searches a hidden spot to pupate, under some beech leafs for example. There he digs a shallow, more or less round little hole in the wet mud. The wall of this hole is egalised and "varnished" with a hardening fluid the larva excretes. The larva the turns on its side in a stooped posture to go in the pupal stage.

Next page : THE PUPAL STADIUM



WEBLINKS:


Photo of a canibalistic larva

Further literature used:

Biologie der Süsswasserinsekten (1943) - Wesenberg Lund
De Waterbeetles van Nederland - Drost, Guppen, van Nieukerken en Schreijer (published by KNNV Netherlands)

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COPYRIGHT:
All pictures on this site were made by Gerard Visser (Almelo, Netherlands), unless stated otherwise. All rights remain with him. These pictures may not be used for purposes any other than private viewing or printing. Do NOT hardlink to these pictures or place them on other websites without the author's approval. Should you need them for purposes which include third parties, you must ask the author permission by e-mail. People, who want to use this pictures for exhibitions or publications or educative material are much encouraged to do so, after approval as mentioned and giving the normal credits.
© G.H. Visser 23-10-2007
rev. 07-07-2008


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