## 4 Discussion  
The volatile blends emitted from cattle dung were principally constituted by \(p\)-cresol, followed to a lesser extent by phenol, indole, terpen, long-chain branched alcohols, and branched and unbranched hydrocarbons, among others. Marked differences in volatile profiles from dung samples were not observed. However, taking in account the significant differences between the behavioral responses of horn flies exposed to volatiles from fresh and aged feces, further studies focused on the temporal variability in dung volatile emissions would be valuable. The compositions of dung blends observed here were representatives of common VOC emanations from the manure of grazing animals, including cows, horses, sheep, and goats [102]. In these volatile blends are various chemical groups of VOCs, including aromatic compounds (phenols and indoles), terpen (isoprene, limonene, etc. ), oxygenated compounds (e.g., cresol, acetaldehyde, etc. ), sulfur compounds (methanethiol, dimethyl sulfide, etc. ), and halogenated compounds, among others [102]. In the olfactory tests using a \"Y\"-tube olfactometer, volatiles from fresh dung significantly attracted mixed-sex horn flies, males and females. Moreover, in the oviposition tests, the egg number laid per female was always greater on the pieces of full covering cattle dung samples than that on blanks. Considering the flies were separated from the feces by pieces of tuple, the attraction and the egg-laying had to be elicited by volatile cues emanated from cattle feces, which passed across the tule and were received by the flies. Additionally, volatiles from aged dung were neither attractive nor repellent to both horn fly sexes, and the egg-laying was minor over the aged dung, but it was not null. Therefore, fresh cattle dung was preferred by flies of both sexes and by female flies for egg-laying rather than aged dung. Nevertheless, the flies had the capability to select and to egg-lay on aged cattle dung anyway. It has been described in literature that most horn fly oviposition occurs in the first 2 min after excretion [1, 103, 104, 105]; notwithstanding, these reports were based on detailed field observation, but the exact period of dung attraction has not been experimentally determined, until now. The results of this study are in agreement that the horn fly oviposition occurs mainly during the first minutes after excretion but also prove this period is not exclusive; in laboratory conditions, dung attraction to horn flies is conserved until 24 h after excretion, and the egg-laying may even occur in manure pats aged for several days. In relation with the olfactory responses of male horn flies, despite the attraction of male flies to volatiles from fresh dung that was observed here, it has not been documented in the literature if males are naturally interested in cattle manure pats in field. Nevertheless, the ecological approach taken to read these results should be weighed with special care regarding limitations inherent in the laboratory work. The olfactory bioassays did not consider visual and other non-visual cues. Moreover, the starved flies used in these analyses were constantly exposed to a light source during the olfactory tests. These conditions do not exactly represent natural conditions in which wild horn flies develop their life cycle. Thus, field studies would be valuable to complement these findings. This study demonstrates, for the first time, the role of volatile semiochemicals emanated from cattle dung in conferring, in part, the attractiveness of it to horn flies, and in stimulating the oviposition of female horn flies, without considering visual and other non-visual cues. Notwithstanding the aforementioned, there is not more information in the literature about the role of semiochemicals emitted from cattle dung in dung selection and in the oviposition of the _H. irritans_. However, it has been reported that horse and cow manure were attractants to stable flies, especially females searching for oviposition sites [97; 98].