<i>Drosophila melanogaster</i> larvae generate force to counteract external mechanical pressure

To counteract or to retreat presents a fundamental dilemma for biological organisms when facing adverse abiotic environmental conditions. In many cases, the predominant strategy animals adopt is to retreat. However, whether counteraction is possible and how the choice between counteraction and retreat is decided are not clear. Here, we report that Drosophila melanogaster larvae can actively counter external mechanical pressure, inspired by Drosophila larval cleft-squeezing behaviour. We developed a behavioural paradigm to investigate the counteracting force of larvae in response to external pressure. Instead of retreating by crawling backward, some D. melanogaster larvae could crawl forward and act against the external physical pressure. Under externally applied forces of 25 mN, 93.9% of forward peristaltic movements increased the counteracting force, while 88.2% of backward peristaltic movements decreased it. The active nature of the counteracting force was reflected by a longer inter-wave delay, more oscillation work and a longer force wave period during consecutive forward peristaltic waves. As the external force was increased from 25 mN to 50, 75 and 100 mN, counteraction by forward peristalsis became less frequent, while retreat by backward peristalsis was more frequent. A reduction of the external pressure immediately following the counteracting forward peristalsis, which might serve as rewarding signal, reinforced the counteraction and induced more forward peristalsis. The rewarding effect of reducing external pressure by forward crawling was much greater than that produced by backward crawling. Our study sheds light on the intricate mechanisms underlying animal proactive responses to adverse abiotic environmental conditions.