Why are there no small species among xylem-sucking insects?

Abstract

Xylem-sucking herbivores must cope with negative tension and extremely low nutrient concentration of xylem fluid. We test the hypothesis that the energetic cost of xylem fluid extraction grows with decreasing body size, as small species have relatively high metabolic rates and small feeding apparatus. The suction pressure gradient needed for feeding (P), calculated using Poiseuille's Law, and the cibarial pump load (L), estimated as xylem fluid volume per unit volume of cibarial muscles, were used to quantify energetic costs associated with feeding. Interspecific relationships between body size and frontoclypeus (cibarial pump) volume, proboscis length and food canal diameter were quantified using the structural relations model and model I (‘ordinary’) regressions applied on species as data points (phylogeny ignored), and on independent (intrageneric) contrasts (taxonomy used as a surrogate of phylogeny). All the dimensions of feeding apparatus changed approximately isometrically with body volume. In consequence, the suction pressure gradient P∼ (body volume)−0.21, and the cibarial pump load L∼ (body volume)−0.17. These relationships result in an allometric change in energetic costs associated with xylem feeding. Energy needed to overcome the resistance of feeding apparatus (P) becomes negligible for any species longer than about 8mm, and there is an important decrease in the L values in species over about 17mm in body length. These allometric trends can result in large species having a wider range of potential (energetically profitable) host plants than small species. Analysis of body size distributions in xylem-feeding insects, which all belong to the Auchenorrhyncha (Hemiptera), showed that xylem feeders tend to be larger than phloem feeders, especially in terms of the minimum body size. Two evolutionary transitions from phloem to xylem feeding in Hemiptera were associated with an increase in the minimum body size. A possibility that the evolution of minimum body size in xylem feeders is energetically constrained is discussed.