WHY ARE THERE NO SMALL SPECIES AMONG XYLEM-SUCKING INSECTS

Xylem-sucking herbivores must cope with negative tension and extremely low nutrient concentration of xylem fluid. We test the hypothesis tha t the energetic cost of xylem fluid extraction grows with decreasing b ody size, as small species have relatively high metabolic rates and sm all feeding apparatus. The suction pressure gradient needed for feedin g (P), calculated using Poiseuille's Law, and the cibarial pump load ( L), estimated as xylem fluid volume per unit volume of cibarial muscle s, were used to quantify energetic costs associated with feeding. Inte rspecific 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') regress ions applied on species as data points (phylogeny ignored), and on ind ependent (intrageneric) contrasts (taxonomy used as a surrogate of phy logeny). All the dimensions of feeding apparatus changed approximately isometrically with body volume. In consequence, the suction pressure gradient P similar to (body volume)(-0.21), and the cibarial pump load L similar to (body volume)(-0.17). These relationships result in an a llometric change in energetic costs associated with xylem feeding. Ene rgy needed to overcome the resistance of feeding apparatus (P) becomes negligible for any species longer than about 8 mm, and there is an im portant decrease in the L values in species over about 17 mm in body l ength. These allometric trends can result in large species having a wi der range of potential (energetically profitable) host plants than sma ll species. Analysis of body size distributions in xylem-feeding insec ts, which all belong to the Auchenorrhyncha (Hemiptera), showed that x ylem feeders tend to be larger than phloem feeders, especially in term s of the minimum body size. Two evolutionary transitions from phloem t o xylem feeding in Hemiptera were associated with an increase in the m inimum body size. A possibility that the evolution of minimum body siz e in xylem feeders is energetically constrained is discussed.