An information-theoretic analysis of the tsetse fly (Glossina morsitans) antenna reveals trichoid sensilla as the main carriers of odor identity

Abstract

Tsetse flies locate vertebrate hosts largely by smell, and odor-baited devices are central to tsetse and trypanosomiasis control. A published electrophysiological survey of the Glossina morsitans antenna defined seven classes of olfactory sensilla, but descriptive clustering cannot reveal how much information these classes carry or which odors they distinguish. We reanalyzed responses of 182 sensilla to 16-17 odorants using Fisher-Rao information geometry, bias-corrected mutual information, Bayesian decoding, and signal detection theory. In the fruit fly Drosophila melanogaster, trichoid sensilla are largely dedicated to pheromones. In tsetse, the three trichoid classes carried more information about odor identity than the four basiconic classes (1.92 vs. 1.14 bits, of a possible 4), a difference of 0.78 bits (95% confidence interval 0.44-1.10), and were strongly excited by the host attractant 1-octen-3-ol. Adding trichoid to basiconic sensilla raised decoding accuracy from 32% to 70% (23% to 55% for decoders tested on held-out sensilla) and identified 1-octen-3-ol on 98% of trials (92% held out). The basiconic code was nearly one-dimensional (participation ratio 1.71), organized along an axis of response strength with 1-octen-3-ol at its extreme. The CO2-sensitive ab4 class was the most sensitive detector for 11 of 16 odorants, whereas ab2 carried no detectable odor information. Classes shared up to 0.41 bits of their information with one another, an overlap only weakly predicted by the similarity of their mean tuning. On a shared odor panel, Drosophila receptors carried twice the information per channel and, with channel number matched, spanned about 1.5-fold more dimensions. These conclusions held in simulations at the real sample sizes, under alternative noise models, and when classes were defined on independent odorants. The tsetse antenna thus combines redundant, detection-oriented basiconic channels with trichoid sensilla that carry host-odor identity, principles relevant to the design of odor baits.