The non-phoretic stages of mites of the genus Hemisarcoptes are predators o
f the family Diaspididae. The heteromorphic deutonymph (hypopus) maintains
a stenoxenic relationship with beetles of the genus Chilocorus. The mites a
ttach to the subelytral surface of the beetle elytron during transport. The
re is variation in mite density among species of Chilocorus. Both Hemisarco
ptes and Chilocorus have been applied to biological control programmes arou
nd the world. The objective of this study was to determine whether subelytr
al ultrastructure (spine density) plays a role in the evolution of symbiosi
s between the mite and the beetle. The subelytral surfaces of 19 species of
Chilocorus and 16 species of Exochomus were examined. Spine density was de
termined for five subelytral zones: the anterior pronotal margin, medial ce
ntral region, caudoventral tip, lateral distal margin and epipleural region
. Spine density on the subelytral surface of Chilocorus and Exochomus was i
nversely correlated with the size of the elytron for all zones except the c
audoventral tip. This suggests that an increase in body size resulted in a
redistribution of spines and not an addition of spines. The pattern of spin
e density in Exochomus and Chilocorus follows a single size-density traject
ory. The pattern of subelytral ultrastructure is not strictly consistent wi
th either beetle phylogeny or beetle allometry. The absence of spines is no
t correlated with either beetle genus or size and species of either Chiloco
rus or Exochomus may be devoid of spines in any zone, irrespective of body
size. A general difference between species of Chilocorus and Exochomus is t
he fact that while spine density in Chilocorus is clinal relative to the si
ze gradient, Exochomus is dichotomous and likely to have either many spines
or no spines in a particular zone. No species of Chilocorus was completely
devoid of spines. Five species of Exochomus had no spines at all, thus mak
ing it difficult to interpret the primary function of the subelytral spines
in a general way. Within the genus Chilocorus, spine density may play a sy
nergistic role in host association. Based on morphological evidence alone,
these findings lead to the hypothesis that the species of Chilocorus that w
ould be most conducive to biological control application in conjunction wit
h Hemisarcoptes would be Chilocorus cacti, Chilocorus distigma, Chilocorus
fraternus, Chilocorus orbus, Chilocorus tristis and, to a lesser extent, Ch
ilocorus bipustulatus. Exp Appl Acarol 23: 97-118 (C) 1999 Kluwer Academic
Publishers.