Since trehalose in insects, in contrast to glucose in mammals, does not enter the haemolymph directly from the digestive tract, but is all synthesized by the insect itself, and furthermore an increased trehalose synthesis during stress and flight does not lead to significant increases in haemolymph trehalose, there seems to be no physiological need for an insect homeostatic hypotrehalosaemic hormone. Experiments in which tissue extractions were found to lower haemolymph trehalose can not prove the existence of such a hormone, while all insect species which so far have been submitted to a trehalose-tolerance test, decrease their haemolymph trehalose concentrations at a rate which can be accounted for by the metabolic use of trehalose. These results therefore indicate the absence, and not the presence, of a homeostatic hypotrehalosaemic hormone. This is also true for blowflies, from which an insulin-like immunoreactive peptide has been isolated. It seems therefore unlikely that this insulin-like peptide is a homeostatic hypotrehalosaemic hormone. The physiological mechanism by which this insulin-like peptide would have to act to function as a hypotrehalosaemic hormone is also an unlikely one. It therefore seems justified to conclude that so far, homeostatic hypotrehalosaemic hormones have not been demonstrated in insects. Furthermore, it may well be that they do not exist.