Journal of Insect Behavior, Vol. 20, No. 2, March 2007 ( C  2007) DOI: 10.1007/s10905-007-9080-5 The Significance of the Vestibular Cell in Trap Nesting Wasps (Hymenoptera: Crabronidae): Does its Presence Reduce Mortality? Josep D. As´ ıs, 1,2 Amparo Ben ´ eitez, 1 Jos ´ e Tormos, 1 Severiano F. Gayubo, 1 and Miguel A. Mart´ın Tom´ e 1 Revised January 12, 2007; accepted February 15, 2007 Published online: March 8, 2007 In a study carried out with the trap-nesting solitary wasp Trypoxylon attenuatum (Hymenoptera: Crabronidae) in 2000, we analysed the signifi- cance of different nest characteristics, like the vestibular cell and empty in- tercalary cells, and its role in the reduction of mortality. Analysis of the presence of the vestibular cell suggested that it represents a nest comple- tion structure, whose presence reduces mortality in the initial stages (M1) but does not affect that due to chrysidid parasitoids (MP). The length of such vestibular cell was found to depend mainly on the space between the last provisioned cell and the exterior, pointing to a possible role in the re- duction of the remaining space that could be used by other females to es- tablish a nest exterior to the completed one. The empty intercalary cells were smaller than the vestibular cells, did not affect M1 or MP mortal- ity rates in the nest, and exhibited an “aggregated” pattern of occurrence. This suggests that they could be the result of “abnormal” behaviour by the female owning the nest. The total mortality rate decreased with the in- crease in the number of cells in the nests, also being greater in nests es- tablished at low heights above the ground. M1 was lower in the innermost cells, while MP increased towards the interior and in the nests with more cells. The distribution in the nests of cells parasitized by Trichrysis cyanea (the main natural enemy found in the wasp population studied) reflects an “aggregated” pattern, suggesting repeated attacks against certain nests. KEY WORDS: vestibular cell; intercalary cell; mortality; Trypoxylon; Crabronidae. 1 ´ Area de Zoolog´ ıa, Facultad de Biolog´ ıa, Universidad de Salamanca, 37071 Salamanca, Spain. 2 To whom correspondence should be addressed ´ Area de Zoolog´ ıa, Facultad de Biolog´ ıa, Universidad de Salamanca, 37071 Salamanca, Spain; e-mail: asis@usal.es. 289 0892-7553/07/0300-0289/1 C  2007 Springer Science+Business Media, LLC