Corixa, abdomen

Sigara falleni, abdomen with enveloping air bubble

The abdomen of the lesser water boatman is relatively small. Yet you might call it the ship's kettle of the insect because the digestive system is situated here, together with other organs. On the picture from left to right: the middle and hind legs protruding through the air bubble, then the abdomen, beige en oval. Notice the small size? The enveloping air bubble is like a streamlining scale in which the abdomen is placed. That air bubble is the supply of breathing air, which is refreshed in a fraction of a second at the water-surface. During the long submerged status of the insect the bubble gets smaller, and the oxygen in it gets depleted. However there is a positive physical effect: when the oxygen percentage in the bubble is smaller than that of the surrounding water, there is an oxygen flow from the water into the bubble. This is called a physical lung.

Note the micro-organisms glued between the hairs at the pointed aft. Water insects are constant pestered by all kinds of attaching creatures, comparable with marine growth on the hull of a ship. The micro-organisms take profit of the mobility of the insect and of food particles floating around it and as usual are merely a nuisance because they somewhat enlarge the water resistance. More trouble is caused by some watermite species, which bite themselves fixed to the back on the abdomen of the Lesser Water boatman and transfer into a parasitic, legless growing stage. Not only do they use the insect for an aeroplane but ″in reward″ for their free flight they suck blood and dissolved tissue from their unlucky host. The insect tries hard to scratch off the parasites with it's legs with pegs, but unfortunately some watermites are fully specialized for their job and may crawl under the wings for example... This site has nice pictures of watermites on water insects.

Corixa species

On the most left picture the body of a deceased Lesser Water boatman, back side. Next to it a drawing of the different parts. From the top: the middle and hind part of the breast, the mesothorax and metathorax are still present and are covered by the little shields mesonotum and metanotum. The (removed) front wings were attached to the mesothorax, the hind wings to the metathorax. Then the abdomen. It is divided in segments. The symmetric division means this Corixa was a female. The drawing on the right shows an example of the asymmetrical division of abdomen of the male. This division forms a groove on the abdomen, which plays a part during mating. The back shields of the segments are called tergites, the sides are called connexivum. The connexivum is somewhat broadened in the epipleura along the side borders. Together with the somewhat half pipe shaped front wing borders they form two flat air tubes: the ″silver striping″ of the Lesser Water Boatman. In the abdomen also the two main airducts or trachea are situated. They are kept open by a supporting spirale, in much the same way as a vacuum cleaner tube is. These airducts branch into smaller ones which have openings (stigmata) on the segments.

Rhoicosphenia
Detail of left

The aft point of the abdomen of Sigara falleni is shown on the larger picture on the left. The long hairs may stabilise the insect while swimming, like the feather of an arrow. The might also have a function in the air replenishing action at the water surface, in holding the insect's body horizontal. On the picture you can further see that the right wing covers the left partially and that the tail lobes protrude a little from under the front wings. On the margin are two microscopic small things, possibly silicon algae of a Rhoicosphenia species, and are pictured again enlarged. This is a species which may often be found on water bugs. The same two specimen again, seen from the under side, on the two pictures below, the second picture being a detail crop of the lower one, which may be enlarged with a mouse click on one of the pictures.. You can see a picture of the tail lobes of a Corixa species, taken through a microscope, by clicking on the right picture below.