The circadian locomotor rhythm of Hemideina thoracica (Orthoptera; Stenopelmatidae): a population of weakly coupled feedback oscillators as a model of the underlying pacemaker

Time measurement in insect photoperiodism: external and internal coincidence

The identity and nature of the photoperiodic photoreceptors are now quite well known, as is the nature of the endocrine regulation of the resulting diapauses. The central problem of time measurement-how the photoperiodic clock differentiates long from short days-however, is still obscure, known only from whole-animal experiments and abstract models, although it is clearly a function of the insect circadian system. This review describes some of these experiments in terms of oscillator entrainment and two widely applicable photoperiodic clock models, external and internal coincidence, mainly using data from experiments on flesh flies (Sarcophaga spp) and the parasitic wasp, Nasonia vitripennis.

Control systems models for the circadian clock of the New Zealand weta, Hemideina thoracica (Orthoptera: Stenopelmatidae)

The New Zealand weta, Hemideina thoracica, is a nocturnal orthopteran insect which emerges from holes in trees or from under bark soon after sunset to forage for several hours on plant and animal material before returning to its refuge before dawn. In tests of the internal clock hypothesis it exhibits clear circadian locomotor rhythms in which the period is initially somewhat less than 24 h, but frequently spontaneously increases to over 25 h. The rhythms are entrainable by light and temperature cycles, obey Aschoff's Law and are temperature compensated. A single oscillator feedback model accounts for these basic properties of the weta clock, but does not explain a variety of examples of rhythm lability, such as day skipping, spontaneous change in period, scalloping and desynchrony typically found in the real data. To account for these characteristics the model is expanded into two linked populations of oscillators, which retain the basic properties of the simple model and in addition interact through their coupling to show the various types of free-run lability. To make these control systems models compatible with the molecular interpretation of circadian biology, each of the components in the feedback loop is matched with molecular function and structure.

Localization of the circadian pacemakers of Hemideina thoracica (Orthoptera; Stenopelmatidae)

The location of the circadian pacemakers of the orthopteran Hemideina thoracica (White) has been investigated through observation of the effects of surgical removal of brain tissues (principally optic lobes and tracts) on free-running and entrained locomotor rhythms. Bilobectomy and severance of optic tracts invariably resulted in arrhythmicity, whereas rhythmicity was sustained following unilateral lobectomy, generally with increases in the free-running period (FRP) and decreases in both the active-phase lengths and activity-to-rest ratios of the rhythm. Bilobectomized subjects could be entrained by temperature cycles, but exhibited no transients or residual rhythmicity, indicating that temperature brought about a direct response or masking effect. These results support the hypothesis that the circadian locomotor pacemakers of Hemideina are located within each optic lobe, and that there are no extraoptic centers for the control of the timing of locomotor activity. Although confirmation of the pacemaker role of the optic lobes requires transplantation of the tissues, the conclusion may be drawn by inference from other studies (e.g., Leucophaea maderae--Page, 1983; Gryllus bimaculatus--Tomioka and Chiba, 1986). Light entrainment continued after surgical binding and blackening of the compound eyes and ocelli, supporting the view that direct illumination of neural tissue through the cuticle may be one possible pathway for light entrainment.