Do spiders urinate in the manner we are usually acquainted with; in fact, are there any animals whic

Do spiders urinate in the manner we are usually acquainted with; in fact, are there any animals which don't?

• HOWEVER spiders may urinate, I hope they undo their flies first.

  Peter Barnes, Milton Keynes.

• ALL animals carry out processes corresponding to urination, but they do this in many different ways. The major functions are: removing products containing nitrogen, removing unwanted salts, and adjusting water balance. In man and mammals all three functions are carried out by the kidneys and a liquid urine is stored in a bladder before voiding. This is possible because water is available and nitrogenous waste is produced as urea, which is soluble and relatively non-toxic. Spiders and many insects convert nitrogenous waste to a very insoluble substance, uric acid, from which almost all water is removed before it is shed as a solid, and it may be argued that this is because they have to conserve water. But birds and flying insects with ready access to water, produce an almost dry 'urine' with uric acid, and it may be that reducing weight by eliminating a bladder containing water, is the more important factor. The functions carried out by our kidneys may be separated in other creatures. Some remove salts by special 'salt glands'; freshwater animals have continually to pump out excess water, and may make a virtue of necessity by discharging nitrogenous waste in the water as water dilutes ammonia, which would be poisonous if concentrated in a bladder. The process can change in the life of a single animal: the larva of a water beetle produces ammonia, but the flying adult changes to uric acid. Bee and antlion larvae store up all their nitrogenous excreta and void it only when they become adult. One animal appears not to excrete nitrogen at all: the greenfly. It puts its nitrogenous waste in its offspring. At first sight this seems an impossible solution; but if it puts one 10th of its production in each of 10 offspring, and an offspring puts one tenth of its own production plus one tenth of the inherited material, ie one hundredth, in each of its children, and so on, it soon arrives at an almost constant load of insoluble uric acid. At the end of the summer aphids lay eggs, and the uric acid in the corpse is broken down and returned to plants from which they obtained the nitrogen in the first place.

  (Sir) James Beament, Queen's College, Cambridge.

• THE MOST concentrated nitrogenous waste is produced by woodlice which are terrestrial crustaceans. They puff gaseous ammonia into the atmosphere.

  (Ms) Mo Killip, Cambridge.