## 4 Discussion  
Our chemical analyses revealed the presence of nerol, neryl formate, pentadecane, (6Z,9Z)-6,9-heptadecadiene, and (_Z_)-8-heptadecene in the headspace of both sexes of the European house dust mite _D. pteronyssinus_ and neryl propionate in the headspace of males only. Because the studied mites were undisturbed, we are confident that these compounds are not related to alarm communication or defense. From these compounds, neryl formate, (6Z,9Z)-6,9-heptadecadiene, and (_Z_)-8-heptadecene were already identified in earlier studies [8,10] but no function was suggested for the two hydrocarbons. Their similar structure, obvious from the fact that (_Z_)-8-heptadecene can also be termed (_Z_)-9-heptadecene, indicates a common biosynthetic origin. Nerol and neryl propionate have not been found in the headspace of _D. pteronyssinus_ so far. Interestingly, no volatiles were found in the headspace of tritonymbs, supporting the idea that the release of pheromones in _D. pteronyssinus_ is stage specific, as has been reported also for other mite species [13,14]. In our studies on the biological activity, pentadecane and neryl propionate were shown to be attractive to males, females, and tritonymbs, (_Z_)-8-heptadecene was attractive only to males and the other compounds caused no reaction. Thus, pentadecane and neryl propionate match the definition of aggregation pheromones, which cause aggregative behavior in conspecifics of both sexes or in the same sex as the emitter [28]. In fact, house dust mites have been reported to form aggregations [29]. Generally, aggregations of arthropods are considered to enhance mate finding, to facilitate feeding, to improve survival under adverse environmental conditions and to provide protection from enemies [6,28]. Due to the fact that tritonymbs reacted to both substances, a function of aggregations in the context of mate finding seems unlikely for _D. pteronyssinus_. The idea that mite feeding is improved by living in aggregations can also be excluded. House dust mites feed on pollen, spores of microorganisms, fungal mycelia, bacteria, plant fibers, lepidopterous scales, animal dander, and skin scales of birds [30]. It is very unlikely that consumption on these food sources is density independent. Rather, aggregations could protect mites from desiccation [31] because water loss reduction was demonstrated in aggregations for the American house dust mite [32]. Likewise, aggregations might provide protection by the \"safety in numbers\" strategy against the natural enemies of the mites, e.g., predatory mites,  
Figure 5: Mean number of adults, nymphs, and total (+S.D., n = 9) of _D. pteronyssinus_ in test (shaded bars) and control Petri dishes (white bars) of a long range olfactometer after 48 h. The test Petri dish was baited with neryl propionate. *** \(p<0.001\) (Whitney-Mann U-test). silverfish, dust lice, or pseudoscoporipones [30]. In addition, the pheromone might also serve several functions at the same time, e.g., to provide protection against desiccation and against natural enemies. The fact that only males but not females reacted to (Z)-8-heptadecene strongly indicates a function as sexual pheromone for this compound. Surprisingly, however, this compound is released not only by females, but also by males. One explanation for this apparent contradiction could be provided by studies in other animal species, likewise showing the release of female sexual pheromones in young males. In the rove beetle _Aleochara curtula_, this is interpreted as an attempt by young males to avoid aggressions by older males [33]. In the parasitoid wasp _Lariophagus distinguendus_, young males are discussed to release female pheromones to distract older males to obtain more matings [34]. Possibly, the release of (Z)-8-heptadecene by males serves a similar function in _D. pteronyssinus_. Thus, the present study identified pentadecane and nervl propionate as two new aggregation pheromone components for _D.