1962] Bush — Genus Anastrepha 97 
Cytotaxonomic Key to the Metaphase Plates of the Common 
Species of Mexican Anastrepha (males only) 
i. All chromosomes evidently acrokinetic (MCA=I2) 2 
— All chromosomes not all acrokinetic (MCA^^ or more) . ... 4 
2. No heteromorphic chromosome pairs present at metaphase 
A. mombinpraeoptans , A. f rater cuius , A. distincta 
— Heteromorphic chromosome pair present 3 
3. Small dot Y chromosome present (Y<o.5 X) A. ludens 
— Rod-shaped Y chromosome present but shorter than 
X chromosome (Y>0.5 X) A. zuelaniae 
4. MCA=i4 to 16 5 
— MCA=i7 to 22 6 
5. MCA=I4 A. spat ul at a 
— MCA=i6 A. striata 
6. MCA=20, XiX 2 Y sex mechanism present (2n=n) 
A. serpentina 
— MCA — 22, secondary constriction on X chromosome 
A. aphelocentema 
Though cytological evidence per se is not always suitable for dis- 
tinguishing some species of Tephritidae, it can support and elucidate 
certain phylogenetic relationships within the group. The cytogenetics 
of certain Diptera has been extensively studied in the past three or 
four decades so that many of the mechanisms of chromosome evolu- 
tion in this group are now fairly well understood (Patterson and 
Stone, 1952; White, 1954; da Cunha, i960). Since the number 
of species of Tephritidae so far investigated is extremely limited, it 
is as yet impossible to determine any conclusive generic or interspecific 
relationships, though some interesting possibilities do appear, par- 
ticularly at the interspecific level in the genus Anastrepha. 
It is possible that A. mombinpraeoptans , A. distincta, A. ludens, 
A. zuelaniae, and the Mexican and Brazilian forms of A. fraterculus 
may form part of a chromosome complex representing a subgenus or 
species group within the genus Anastrepha. This is supported by the 
similarity in the morphology of the adults of these species. The 
difference in karyotypes between the Mexican form of A. fraterculus 
reported here and the Brazilian population described by Mendes is 
interesting since this difference may represent a case of chromosomal 
polymorphism or, more likely, sibling species. Biological data support 
the latter (A. C. Baker et al., 1944; E. W. Baker, 1945) in that 
slight but consistent morphological differences exist in the adults 
