Eg: Holotype of Xenocatantops humilis (Serville, 1838) is of lost status. Whereas, Caloptenus strictus (Walker, 1870) a synonym species of the above mentioned still has its holotype in BMNH. So while considering as a next main reference to the former what type should the latter be labelled if recruited to the place of the lost holotype.
Type specimens are associated to names, not to biological entities (such as species, genera, etc.), and are therefore referred to as "name-bearing type specimens". When no name-bearing type specimen (i.e. holotype, lectotype, syntype or prior neotype) is believed to be extant, a neotype may be designated (ICZN Article 75). Consequently, in your example, the holotype of Caloptenus strictus cannot be transferred to Xenocatantops humbles, regardless of the fact that the latter is a junior synonym of the former.
Type specimens are associated to names, not to biological entities (such as species, genera, etc.), and are therefore referred to as "name-bearing type specimens". When no name-bearing type specimen (i.e. holotype, lectotype, syntype or prior neotype) is believed to be extant, a neotype may be designated (ICZN Article 75). Consequently, in your example, the holotype of Caloptenus strictus cannot be transferred to Xenocatantops humbles, regardless of the fact that the latter is a junior synonym of the former.
Yuri is right. If the holotype is lost, you first need to find out if there are any isotypes. If so, you may designate one of the isotypes (preferably one mentioned or annotated by the author) as the lectotype.
If there is no isotype either, check if there is any other collection that the author of the original name has used when he described the species (= a paratype). If so, you may designate one of the paratypes as lectotype.
If there is no paratype either, then an illustration in the original publication may serve as lectotype.
If there is no illustration either, look for specimens annotated by the original author that are as closely in agreement with the protologue as possible. One of these would be a good choice for a neotype, which may be selected if there is no original material at all.
If there is no material at all that can safely be assumed to represent the original concept of the species, then you are free to select any specimen as neotype, but of course you should take care that it agrees with the protologue.
As Jens implicitly points out, if the situation is such that a neotype is advisable, then you are free to select the holotype of another nominal species as neotype. But you'll have to be very careful about it. Point is that a holotype cannot lose its status, but there is nothing that prevents a single specimen to be the holotype or neotype of two or more valid nominal taxa, which then are objective synonyms.
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