Taste hairs are located on the labellum and tarsi of blowflies. These multimodal hairs consist of four functionally distinct chemoreceptors and a mechanoreceptor. By staining selected multimodal hairs, we sought to identify the central projection patterns of multiple and single axons from those hairs. On each side of the labellum there are 11 "largest" hairs (LH). The neurons associated with the anteriormost (LH-1), posteriormost (LH-11), and one lateral (LH-6) hair on the labellum were stained selectively with cobaltous sulfide. The overall projection pattern in the central nervous system (CNS) for axons from LH-1 and LH-11 is similar and differs markedly from axons from LH-6. At least three individual axon-projection patterns were determined for each labellar hair filled, indicating a partial functional organization for axons from multimodal hairs. One identified axon, the dorsalmost axon, has terminal arborizations that do not differ with the location of its associated hair. Another axon, thicker than the others, projects to a region that is distinct from the four thin axons. Within this region the arborizations of the thick axons occupy different areas depending on the location of their associated hair. Neurons from the largest hairs on the distalmost tarsomere (D5) of each leg were also stained and consisted of one thick and four thin axons. All axons except one thin axon from tarsal D5 hairs terminate in their respective leg neuromeres. The remaining thin axon projects to the suboesophageal ganglion ipsilateral to the hair filled and terminates in the same region as a branch of the labellar dorsalmost axon. These data suggest that axonal arbors from multimodal hairs have a limited functional and somatotopic organization in the blowfly CNS.

A key adaptation for any parasitoid insect is the sensory modality that it uses to locate its host insect. All members of the speciose family Tachinidae (Diptera) are parasitoids, but only flies of the tribe Ormiini use acoustic cues to find their hosts. Ormiine flies are parasitoids of various genera of crickets and katydids. Gravid females of one ormiine species, Ormia ochracea, hear the reproductive calling song of male field crickets and home in on those calls to locate their hosts. While many flies possess various kinds of "ears" to detect airborne sounds, only ormiine flies have been reported to possess true tympanal hearing organs. Such organs are well-known to occur in their cricket and katydid hosts. The ormiine ear is an evolutionary innovation within Diptera. Our objective was to trace the phylogenetic origins of the tympanal hearing organ among higher flies. Since the ormiine hearing organ is a complex organ within the prothorax, we examined possible precursor structures in the prothoraces of selected Diptera. We have uncovered a suite of characters that define the ormiine ear. These characters in the prothorax include a pair of prosternal tympanal membranes, a pair of chordotonal sensory organs, and modifications of the tracheal system. We have been able to identify and trace the presumptive homologs of these ormiine characters through selected species of related Diptera, using the method of outgroup comparison.

Tympanal hearing organs have been reported only recently for Diptera. All the cases documented so far relate to parasitoid tachinid flies of the ormiine tribe. In the ormiine flies, the presence of tympanal hearing is functionally linked to their reproductive behavior. Indeed, female ormiine flies detect and localize their host, typically singing orthopterans, by hearing their calling songs. The three ormiine fly species investigated here at the comparative level share the key morphological features associated with tympanal hearing. The extent of these structural modifications becomes evident in the light of comparison with the closely related atympanate tachinid Myiopharus doryphorae. We document a series of eight characters that constitute specialized modifications of the ventral prothorax: (1) an inflation of the probasisternum, providing a rigid frame to span the large tympanal membranes; (2) an increased surface area of the prosternal membranes that constitute very thin, corrugated tympanal membranes; (3) a forked, broad presternum with tympanal pits to which the sensory organs directly attach; (4) several modifications of the tracheal system comprising the enlargement of the prosternal air sac, a supplementary tracheal tube to the prosternal air sac accompanied by a subpartioning of the spiracular atrium, and larger mesothoracic spiracles; (5) the presence of two scolopophorous chordotonal organs in the unpartitioned prosternal air sac; (6) stiff cuticular apodemes linking the chordotonal organs to the presternum; (7) reduction in size of the cervical sclerites; and (8) several structural modifications of the prosternal apophyses, creating new attachment sites for neck muscles. This comparative approach brings out differences and similarities of the homologous cuticular structures found on the ventral prothorax of both tympanate and atympanate tachinids. It is proposed that, given the degree of similarity between the ormiine hearing organs, the ormiine tribe is monophyletic, whereby all members of this tribe evolved from a common ancestor, an acoustic parasitoid of a singing orthopteran insect.

HPLC with electrochemical detection was used to determine the levels of p-hydroxyphenylethanolamine (octopamine), 3,4-dihydroxyphenylethylamine (dopamine), and 5-hydroxytryptamine (5-HT) in the brains of control, reserpine, and d-amphetamine-treated blow flies, Phormia regina Meigen. Parallel studies were carried out to assess the effects of the two drugs on fly feeding behavior, measured as mean acceptance threshold: the minimum sucrose concentration to which the average fly in a population will respond by proboscis extension when its tarsi contact the solution. In saline-injected control flies, all three amines were found at levels of approximately 2 pmol/brain. Thirty minutes after injection with d-amphetamine (12 micrograms/fly), brain octopamine was depleted by 85%, whereas dopamine and 5-HT were depleted by 70%. Reserpine (5 micrograms/fly) caused 70% depletion of dopamine and greater than 90% depletion of both octopamine and 5-HT 24 h after injection. However, the effect of reserpine was much slower in onset (hours versus minutes) and more persistent (days versus hours) than was the effect of d-amphetamine. With either drug, the time course of amine depletion closely matched the time course of the increase in feeding threshold observed in drug-treated flies. These results suggest that CNS pools of the biogenic amines, octopamine, dopamine, and 5-HT are important in governing blow fly responsiveness to food stimuli.

The regulation of feeding behavior in adult Drosophila melanogaster includes such elements as ingestion responsiveness, volume ingested in a single meal, food storage in the crop and rate of defecation. Our results suggest that feeding behavior varies in a manner dependent on feeding regime (food-deprived or ad-libitum-fed) and nutritional state. Fed flies that are subsequently food-deprived become increasingly more responsive to food stimuli over time and, when offered 1% agar diets containing different concentrations of sucrose, ingest greater amounts of diets that have higher sucrose concentrations. When fed ad libitum for 72 h on these same diets, D. melanogaster maintained much smaller crops on average than food-deprived flies fed a single meal. Additionally, ad-libitum-fed flies are grouped into two categories depending on the concentration of sucrose in the diet. Flies fed for 72 h on 1% agar diets having 50 mmoll-1 sucrose or more are not affected by the concentration of sucrose in the diet, while flies fed on diets of 15 or 25 mmoll-1 sucrose increase ingestion responsiveness, crop size and the rate of defecation with decreasing concentrations of sucrose in the diet. Flies fed on even lower sucrose concentrations (5 or 10 mmoll-1 sucrose) for 27-72 h exhibit both a shift over time to larger crop sizes and increased mortality over those of flies fed 15 mmoll-1 sucrose. These data suggest that flies fed ad libitum are capable of modulating their feeding behavior in response to their nutritional state.