Damaged or missing appendages are a common problem in numerous vertebrate and invertebrate taxa and can negatively affect many aspects of life: mobility, foraging, predator avoidance, growth, and development have all been shown to suffer. Many animals use autotomy to survive a predation attempt but suffer diminished performance until the missing appendage heals or regenerates. Animals that regenerate face an extra problem, as they must re-allocate resources away from growth and development in order to regrow their missing parts. We examined the effect of appendage loss on the growth and development of the damselfly Ischnura posita. Damselfly larvae have three caudal lamellae, external gills that can be autotomized and eventually regenerated and that are frequently missing from animals in natural populations (30-40% missing at least one in our collections). We collected animals from the field and raised them individually in the lab, removing lamellae at different times through ontogeny and predicting that growth rate and development rate would be most affected by earlier injuries. As larvae neared emergence to adulthood, we expected them to devote less resources to regeneration and therefore experience less of a decline in growth and development. 

Results/Conclusions

We found that injuries did not strongly affect larval growth rates or sizes, regardless of timing, but that injury positively affected adult size at emergence. Because reproductive success is positively correlated with adult size in odonates, injury may actually increase expected reproductive success of individuals that emerge successfully. Developmental rates, measured as time per instar, were not strongly affected by injury. A latent effect of injury on survival appeared at emergence, with fewer injured individuals able to emerge successfully. Most larvae attempted to emerge, but injured individuals were about twice as likely to fail, dying when they were unable to exit their exoskeletons. In sum, injured damselflies fared as well or better than their uninjured counterparts in terms of growth and development but faced a much greater chance of death, showing a clear but very risky benefit of autotomy.