A review of various aspects of invertebrate-microbial interactions is presented. Food resources of stream invertebrates are distinguished by particle size (detritus), the presence of chlorophyll (periphyton), and high protein content (macrophytes). Invertebrate functional feeding groups such as shredders, collectors, scrapers, piercers, and predators are discussed. The interactions between temperature and quantity and quality of stream macroinvertebrate food resouces are described. Symbiotic relationships between stream invertebrates and microorganisms are discussed.

Feeding Ecology of Stream Invertebrates

BURG, M.,J. GRANTHAM, M. ABRAMOW, AND J. ORLOFF. Preparation and study of fmgments of single rabbit nephrans. Am. J, Physiol. 210(6): 1293-1298. rg66.-A method has been developed for the dissection and in vitro study of fragments of single rabbit nephrons. Fragments of different tubule segments 3-4 mm long were dissected from set tions of kidney immersed in oxygenated Ringer solution. Electrolyte and water composition of proximal tubule fragments were determined after incubation in appropriate solutions. It was found that isolated proximal tubules maintain large transcellular concentration gradients for Na, K, and Cl similar to kidney slices and mixed suspensions of kidney tubules. In order to measure transcellular transport, individual tubule segments were perfused and changes in the volume and composition of the effluent perfusion fluid measured. The viability of perfused proximal tubules was demonstrated by the presence of both active PAI-I transport and net Auid absorption.

Preparation and study of fragments of single rabbit nephrons.

Water balance in land arthropods

Aëdes aegypti (L.), the yellow fever mosquito

BURG, MAURICE, AND NORDICA GREEN. Function of the thick ascending limb of Henle’s loop. Am. J. Physiol. 224(3) : 659-668. 1973.-The thick ascending limb of Henle’s loop was dissected from rabbit kidneys and perfused in vitro. There was net NaCl absorption from the tubule lumen with resultant lowering of lumen NaCl concentration below that of the bath. Since the electrical PD was positive in the lumen, Cl is absorbed actively. Na absorption is largely passive as indicated by comparison of 1) Na flux ratio and PD (at fast perfusion rates); and 2) Na concentration ratio and PD (at slow perfusion rates). However, the possibility of some active Na transport is not ruled out. The PD results from Cl transport. It is eliminated reversibly by removal of Cl from bath and lumen (SO4 substitution) and by ouabain. The PD is increased by removal of Na from bath and lumen (choline substitution). Na permeability is greater than Cl permeability, as indicated by I) the NaCl diffusion potential that is lumen positive when lumen NaCl concentration is low, and 2) radioisotope measurements of ion fluxes. Water permeability (Lp) is low, which combined with the NaCl transport accounts for the dilution of the urine in this segment.

Function of the thick ascending limb of Henle's loop.

1. Two methods of titrating chloride with Ag + ion using the potentiometric end-point are described. 2. The first method is conventional in that silver nitrate is added from a burette. It deals with volumes down to 0.2µl. and can measure 1µg. of chloride with an error of 3. According to the second method Ag + ion is added by passing a current through a silver electrode in series with a condenser. The charge developed on the condenser is a measure of the amount of chloride titrated. This method deals with volumes down to 0.5 x 10 -3 µl. and can measure 10 -4 µg. of chloride with an error of 4. As far as is known these methods are not susceptible to interference from other substances likely to be present in biological fluids.

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Electrometric titration of chloride in small volumes

1. Single Malpighian tubules of the stick insect have been studied as preparations isolated in drops of haemolymph under liquid paraffin. Measurements of osmotic pressure have been made on haemolymph and urine.

2. The urine is more or less isotonic with the haemolymph over a wide (45-145%) range of osmotic pressure. But in a majority of cases the urine is slightly but significantly hypotonic to the haemolymph.

3. From these and other observations it is concluded that water must be actively transported across the wall of the tubule. The implications of this conclusion are discussed.

4. The normal rate of urine production is of the order of 0.07 x 1o-3 cu.mm./sq.mm./min. The rate increases with increasing dilution of the haemolymph and vice versa.

5. Urine production normally ceases, probably as a result of distension of the tubule, when the pressure in the lumen rises to about 20 cm. H2O.

Active Transport of Water by the Malpighian Tubules of the Stick Insect, Dixippus Morosus (Orthoptera, Phasmidae)

1. The processes of osmotic regulation in the larvae of Aedes aegypti and of A. detritus have been studied by determination of the freezing-point of samples of fluid collected from different parts of the gut.

2. In A. aegypti , kept in fresh water (its normal environment), the fluid passing down the intestine to the rectum is isotonic with the haemolymph. In the rectum it becomes strongly hypotonic before being eliminated.

3. In A. detritus , kept in sea water (its normal environment), the opposite process is observed, the fluid in the rectum becoming hypertonic to the haemolymph and approximately isotonic with the external medium before being eliminated.

4. In A. detritus , which is able to live in dilute media as well as in sea water, the only two specimens from fresh water available for examination were found to have the rectal fluid hypotonic to the haemolymph.

5. The ability of A. detritus , not possessed by A. aegypti , to produce an hypertonic fluid in the rectum is tentatively associated with a region in the anterior part of the rectum and lined with an epithelium distinctly different from that in the remainder of the rectum. This anterior region has not been found in A. aegypti .

Osmotic regulation in mosquito larvae.

1. The concentrations of sodium and potassium in the haemolymph and in the urine have been measured in eight species of insect. 2. The concentration of potassium in the urine is always greater than in the haemolymph. The concentration of sodium in the urine is generally less than in the haemolymph. 3. In seven of the species the difference of electrical potential across the wall of the tubule has also been measured. 4. In these seven species the results lead to the conclusion that potassium is actively secreted into the tubule. It is very probable that the same is true in the eighth case. 5. It seems likely that the excretion of sodium can be brought about by passive diffusion into the tubule.

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Active Transport of Potassium by the Malpighian Tubules of Insects

1. Contrary to expectation, it is found that metabolically useful substances pass into the urine and are reabsorbed in the rectum. 2. The kinetics of penetration have been investigated for six amino acids, three sugars and urea. 3. The evidence indicates that these substances enter the urine by passive diffusion. 4. The role of the Malpighian tubules in the insect9s excretory system is discussed in the light of these findings. It is suggested that the tubule is primarily a means whereby all soluble substances of low molecular weight are removed from the haemolymph and that in this respect it has analogies with the glomerulus as well as with the tubule of the vertebrate nephron.

J. A. Ramsay

Papers

Electrometric titration of chloride in small volumes

Active Transport of Water by the Malpighian Tubules of the Stick Insect, Dixippus Morosus (Orthoptera, Phasmidae)

Osmotic regulation in mosquito larvae.

Active Transport of Potassium by the Malpighian Tubules of Insects

Excretion by the Malpighian Tubules of the Stick Insect, Dixippus Morosus (Orthoptera, Phasmidae): Amino Acids, Sugars and Urea