Referee 2's Review:

General Comments:

The manuscript tests the ecological niche dimensionality hypothesis using Timema stick insects. The authors propose that selection for both crypsis (to avoid predation) as well as physiology (to adapt to different host plants) should result in greater divergence between species than selection in just one trait dimension. Data from field experiments (already published experiments and new ones) are used to test this hypothesis using ecotypes of T. cristinae and T. podura, and the species pair T. podura and T. chumash. The results provide evidence in support of the niche dimensionality hypothesis and suggest that the number of traits under selection needs to be taken into account when investigating and trying to explain the variability in the degree of divergence between species.

In general I think that the manuscript is very tidy and concise, and provides interesting and novel information about the factors that affect the variability in divergence between species. Although the authors only investigate one species pair in detail (which limits the generality of their conclusions), I think that this manuscript will be important in stimulating research in this area.

Minor Comments:

Page 2, Line 21: reference is lacking after '...following speciation...'
Page 3, Line 26: delete are versus
Page 5, Line 15: T. podura can be found on Ceanothus as well
Page 5, Line 29: Comma before respectively
Page 7, Line 17: result should be results
Discussion: The authors are too general in the beginning of the discussion (they start out saying that these findings provide a model of diversification in Timema stick insects), and I feel that the results are extrapolated beyond a level that is appropriate. Specifically, the statement that (line 18 and 19) 'divergent selection on the single dimension of crypsis appears insufficient to complete speciation' should be modified, as the authors base this statement on limited data. It is unknown if other species pairs in this group follow a similar pattern, for example, in situations when predation is more intense or when host plants vary more strongly in chemical composition or morphology.
Page 8, Line, 25: '...of selection on single niche dimensions does exist...'
Material and Methods, Page 10 and 11: inconsistent writing of dimensions etc. Sometimes the authors add a space and sometimes not (e.g. 12m and 12 m).
Page 11, Line 5: Add the year to the date
Page 14, Line 6: BMC??
Page 15, Line 20: add pages
Page 16, Line 5: add pages
Page 17, Line 7: Delete dot after Funk DJ
Page 17, Line 22: Delete dot after (2005)
Page 17, Line 30: 2nd should be superscript
References: It would be good to include DOI numbers
Page 18, Figure 1, Line 2: Add that the host plants are shown too
Page 19, Figure 1 taxon pairs: The differences in plant morphology are impossible to see. The authors need to use a different photograph here.
Page 19, Figure 1, Line 4: How were the genealogies derived? A summary of the methods would be necessary.
Page 2, Figure 2, 1 Line 4: The term bugs is too general and should be replaced
Page 23, Figure 3: The graphs showing divergence of populations needs to be improved

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.