Behavioral repertoire of Drosophila larvae

Orientation behaviors are usually classified into two groups: *kinesis* and *taxis*. Kinesis involves undirected changes in activity in function of stimulus intensity. A process is categorized as *klinokinesis* when the rate of turning is a function of stimulus intensity. In *orthokinesis*, the rate of movement (activity) is a function of stimulus intensity. For both orientation strategies, migration toward (or away from) the source of an attractive (or repulsive) stimulus results from a series of undirected reorientation events. A well-documented example of klino kinesis is bacterial chemot- axis. Escherichia coli navigate chemical gradients according to a biased random walk (a klinokinesis). In short, bacterial loco- motion is based on the alternation between two modes of motion: straight runs and random turns. When a bacterium experiences increasing concentrations of an attractant (e.g., sugar), it tends to suppress turning (positive chemotaxis: Δ C /Δ t > 0 leads to turn suppression). In all other conditions, turning occurs at a constant rate. The same principle governs negative chemotaxis: turning is suppressed when a concentration decrease is experienced (negative chemotaxis: Δ C /Δ t < 0 leads to turn suppression).  
In contrast to the indirect orientation strategy featured by kinesis, taxis involves directed orientation based on spatiotemporal comparisons of stimulus intensities. In the case of tropotaxis, orientation results from the instantaneous comparison of sensory input transmitted by different sensors that are physically separated. For instance, adult flies, ants, and bees are capable of detecting concentration differences between their left and right antennae. This stereo-olfaction mechanism permits them to orient in the field of an odor gradient. Not all taxis involve comparisons between the inputs of spatially separated sensors. Klinotaxis involves comparison between sensory inputs—snapshots—measured at different time points. For olfactory receptor and photoreceptor neurons located in the anterior part of the body, lateral motion of the head allows a larva to “canvass” its sensory conditions during forward locomotion. Through this temporal sampling mechanism, changes in direction are biased toward or away from the gradient. The same mechanism permits sharks to ascend odor trails. Notably, humans are also able to perform scent tracking by wavering their head either side of a scent trail—the same mechanism observed in insects and other vertebrates. **Orientation of Drosophila larvae is a kind of taxis behavior.**  
In response to light, larvae avoid regions of high stimulus intensities. For attractive odors, locomotion is directed toward high stimulus intensities. The navigation strategy controlling these two types of orientation behavior is currently the focus of much attention. Even though the orientation algorithms directing phototaxis and chemotaxis are not fully understood, they appear to involve a precise assessment of local gradients [2].  

*Chemotaxis*: In Musca , single larvae respond to food odors by orienting toward the odor source and by staying in the vicinity of the odor. Drosophila larvae display a similar directed response. Given that larvae have a pair of bilaterally symmetric DOs, it is reasonable to hypothesize that stereo-olfaction (comparison between left and right olfactory input, tropotaxis) is the strategy they use to orient in odor gradients. Initial evidence suggested that uni-lateral surgical ablation of the DOs led to circling behavior toward the side of the dysfunctional olfactory organ. Another series of experiments employed a probabilistic rescue strategy to obtain unilateral olfactory function. Using this technique, it was concluded that *bilateral olfactory function is not necessary for larvae to chemotax*. Notwithstanding, larvae with unilateral olfactory function showed reduced performance compared to individuals with bilateral function. This finding suggests that *left-right comparisons enhance the signal-to-noise ratio when detecting changes in odor concentration*. Having ruled out a mechanism based solely on stereo-olfaction (tropotaxis), what strategy might be used by larvae to chemotax? Our work indicates that they use an active sampling mechanism to navigate in odor gradients. *During forward locomotion, larvae appear to collect information about the odor gradient by sweeping their head from side to side. Such a mechanism would rely on **decisions involving comparison between odor intensities measured at different points in space (klinotaxis)***. Orientation would result from temporally based decisions. **Unilateral olfacotry function works for orientation, while reducing performance compared with bilateral case.**

*Phototaxis*: During the feeding stage (up to late third instar), larvae are strongly photophobic. Upon sudden exposure to light, larvae stop moving and begin to sweep their head from side to side. This behavior was reported in Drosophila by Mast as early as 1911. He also noted that they made larger movements when sweeping the head away from the light source than toward it. After a series of head sweeps, the larvae tend to orient in the opposite direction to the light source. About 30 years after Mast, Bolwig made the same observation in larvae of Musca. These findings have been confirmed in more recent experiments involving computer-aided tracking. As with chemotaxis, it is reasonable to hypothesize that *larvae detect differences in light intensities during these lateral head sweeps*. Turns would be directed toward the side where the PRs on the head receive less light. This model proposes *klinotaxis* as a mechanism for phototaxis.

As described above, **the navigational algorithms allowing larvae to chemotax or phototax likely rely on temporally based decisions (klinotaxis)**. This process involves a comparison of odor intensities measured at different points in space. The contribution of spatial comparisons between paired olfactory or visual organs (tropotaxis) should not, however, be excluded. This type of process seems to **involve some sort of memory**—a hypothesis yet to be tested in the larva. Concerning the use of bilateral comparisons in the detection of olfactory signals, theoretical arguments predict that the typical concentration differences measured between the two DOs would fluctuate too much to be detected reliably. It is reasonable to speculate though that processing of the same signal by left and right sensors increases the signal-to-noise ratio when detecting minute changes in concentration over time. Such a process would have to involve **cross talk** between the left and right olfactory pathways. Circuits capable of implementing this operation have yet to be identified (**Potential topics to determine the circuit structure**).

Aggression is a well-known trait throughout the animal kingdom. It is essential for a variety of elementary functions like acquisition of food or mates and defense against predators. By contrast, excessive aggression requires high-energy depending acts that are evolutionary unfavorable.  

Feeding is a vital behavior for all organisms. It is necessary to facilitate growth, to insure survival and to meet reproductive requirements. Feeding can be regulated by multiple factors such as metabolic requirements, feeding status and different sensory inputs, including olfactory and gustatory signals. Furthermore, this behavior includes multiple decision-making processes, some obvious, e.g., hungry or not hungry, and others more complex, e.g., choosing between different food sources and determining whether to eat reduced-quality resources with possibly novel or aversive tastes in order to survive.

Phototaxis behavior implies the movement of an organism in the direction of a light source or away from it. **While adult flies are positively phototactic, larvae are photophobic**. These light preferences can be crucial for survival. 1) **Adult phototaxis behavior** is polygenic and is influenced by different factors such as age or rhabdomere structure of the eye. 2) **Light avoidance in larvae** is regulated by the paired Bolwig organs, which form the primitive eye structures in this life stage and which are connected to the pigment dispersing factor expressing lateral neurons. Further, it has also been shown that light sensitive dendrites in the body wall contribute to this behavior as well as two pairs of isomorphic neurons in the central brain which seem to be involved in the decision between light and dark.

Drosophila third-instar larvae exhibit changes in their behavioral responses to gravity and food as they transition from feeding to wandering stages. Using a thermal gradient encompassing the comfortable range (18°C to 28°C), we found that third-instar larvae exhibit a dramatic shift in thermal preference. Early third-instar larvae prefer 24°C, which switches to increasingly stronger biases for 18°C–19°C in mid- and late-third-instar larvae. --[(**age-dependent transitions in temperature selection**)](https://www.sciencedirect.com/science/article/pii/S2211124716312505)

A big step toward a global understanding of larval behavior will be the development of computer-aided automated tracking. ### ***For Drosophila adult*** * [High-throughput ethomics in large groups of Drosophila](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2734963/) * [Automated monitoring and analysis of social behavior in Drosophila]() * [Complex behavioural changes after odour exposure in Drosophila larvae]()